High aspect ratio lithium niobate ridge waveguides fabricated by optical grade dicing

Question

1. What are the main techniques used for etching ridges?

2. What is the way to confine light?

3. What is the way to fabricate ridges?

4. What is the method for etching a ridge?

Accepted Answer

Wet etching techniques associated with ion implantation, proton exchange or domain inversion, have also been widely studied [12-14].

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Another way of vertically confining light consists in exploiting thin LiNbO3 layers fabricated by wafer bonding and thin film forming technology such as Smart Cut.

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Ultrafast laser machining, which is amenable to directly pattern any material with trenches as deep as several hundreds of micrometers could appear as a potential candidate for meeting the requirement ofa quick process.

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If the wet etching process is prepared with an annealing step, and followed by a subsequent step of Ti-indiffusion, the method shows to be very efficient for the fabrication of ultra-smooth ridges with a roughness of 20nm and propagation losses as low as 0.05dB/cm [15,16].

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In the view of evaluating more precisely the propagation losses, the authors have measured the transmitted intensity independently ten times, which has led us to an average estimation of the propagation losses, and to an estimation of the measurements error bar: α=0.50±0.48 dB/cm.

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(a) Visualisation of the light confined between the walls of a 35µm deep, 3.5µm large ridge waveguide for an immersion time of 6 hours at 200°C in melted benzoic acid.

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As a conclusion, optical grade dicing process shows to be an extremely efficient tool for quick fabrication of straight ridge waveguides with smooth walls and high aspect ratios.

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the propagation losses remain relatively low even for thin ridges: they are typically of 2.5±2.4 dB/cm for a ridge with a top width of 3.5µm.

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The authors have tested the influence of a subsequent annealing upon the optical mode in order to reduce the propagation losses inherent to PE waveguides.

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The high number of papers that still deal with the subject shows that there is a current need for a method enabling the fabrication of ridges with low propagation losses, smooth and vertical walls with a process that is not too much time consuming and expensive.

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An optimal structure has been put in evidence with Ti-diffused planar waveguide exhibiting a top width of 6µm and a depth higher than 20µm: in that configuration, propagation losses of 0.5dB/cm can be reached, with a lateral FWHM of 3µm.

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The method applies indifferently to Z-cut or X-cut substrates, for any direction of propagation, and takes less than thirty minutes of process for the fabrication of one ridge.

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These tests have shown us that minimal immersion duration of 12 hours is needed to obtain single mode propagation within a ridge with a top width of 3.5µm.

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As can be seen in the figures 1(c) and 1(d), end facets and sidewalls of the films were free of chipping and delamination, demonstrating that the optical grade dicing method is not limited to homogeneous congruent substrates.

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Thin lithium niobate films may be formed using a process similar to the Smart Cut process used to fabricate silicon on insulator (SOI) wafers [18,19].

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This effect can be overcome by realizing the annealing step before dicing the samples: the authors are currently working on annealed proton exchanges planar waveguides in order to determine the optimal conditions for combining low propagation losses, high electro-optic interaction and optimal confinement of the light.

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The depth is not reported as a key parameter in the table: according to their observations, the influence of the depth becomes negligible if the ridges are deeper than 20µm.

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The impact of the non verticality of the walls is seen in figures 2(b) and (e) and summarized in table 1: the full width at half maximum (FWHM) is reduced 1.9 times when the sidewall angle becomes 88° (figure 2(b)) instead of 40° (figure2(e)).

Accepted Answer

If etched on their sides, standard waveguides can benefit from a higher lateral index contrast, which enables an increased confinement of light and a higher overlap coefficient between the guided optical wave and the modulating microwave signal in electro-optic (EO) devices.

High aspect ratio lithium niobate ridge waveguides fabricated by optical grade dicing

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Most frequently asked questions

1. What are the main techniques used for etching ridges?

2. What is the way to confine light?

3. What is the way to fabricate ridges?

4. What is the method for etching a ridge?

5. How many times have the authors measured the transmitted intensity independently?

6. How long does the ridge waveguide take to be absorbed?

7. How can a ridge waveguide be made?

8. How low are the propagation losses for thin ridges?

9. What is the effect of annealing on the optical mode?

10. What is the current need for a method for the fabrication of ridges?

11. How many dB/cm can be achieved with a Ti-diffused?

12. How long does it take to fabricate a ridge?

13. How long does it take to get a single mode to propagate?

14. What is the performance of the optical grade dicing method?

15. What is the process used to fabricate silicon on insulator wafers?

16. How can the authors overcome the effect of annealing?

17. What is the effect of the depth on the propagation losses?

18. What is the effect of the non verticality of the walls?

19. What is the difference between ridges and planar waveguides?

Figures

Citations

Photonic crystal electro-optic devices in engineered thin film lithium niobate substrates

Tunable Linear Polarization-State Generator of Single Photons on a Lithium Niobate Chip

Optical characterization of ultra-short Bragg grating on lithium niobate ridge waveguide

Acoustic resonator based on periodically poled lithium niobate ridge

Towards proton exchanged quantum wires and highly confining integrated circuits on LiNbO3

References

Frontiers of silicon-on-insulator

Lithium niobate photonic wires

Lithium niobate ridge waveguide modulator

Ion-sliced lithium niobate thin films for active photonic devices

Efficient 494 mW sum-frequency generation of sodium resonance radiation at 589 nm by using a periodically poled Zn:LiNbO3 ridge waveguide.

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