Function-Versatile Thermo-Optic Switch Using Silicon Nitride Waveguide in Polymer
Round 1
Reviewer 1 Report
The authors designed and fabricated a function-versatile optical switch using a combination of phase shiftable MZI and MMI structures. The optimization of 1 × 2 MMI splitter and 2 × 2 MMI for path switching were given. Moreover, according to the measurement results, the device could realize the designed different functions such as polarization independent beam splitting, path switching, and polarization beam splitting. The analysis process is substantial, which has practical application value. This manuscript should be suitable for publication after a minor revision. I have several suggestions listed as follows,
Question 1: The top and bottom cladding thicknesses are 6 μm and 15 μm, respectively. How did the authors select these thicknesses as optimal dimensions for the fabrication of actual waveguide devices.
Question 2: The equipment model of ellipsometer on page 6, line 197 should be marked. In addition, on page 5, line 167, the variable L in Equation 6 was not clearly marked in the manuscript.
Question 3: As illustrated in Figure 2a, the maximal temperature on the micro-heater is 240 °C. In the actual work of the waveguide device, whether the authors consider the problem of device aging and working stability.
Author Response
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Author Response File: 
Author Response.docx
Reviewer 2 Report
The Authors propose a multi-function circuit able to implement beam splitters (BS), path switches (PS) and polarization beam splitters (PBS). The performance has been estimated by using a commercial software (Lumerical) based on FDTD method for the thermal simulation and subsequent light field propagation simulation based on the eigenmode expansion method. Although the idea is interesting for the next generation programmable photonics, a major revision of the manuscript is suggested to address the following comments:
- In the Introduction Section, the Authors should discuss more about the potential applications and their target performance to help the reader in the device rating. Moreover, the discussion on the competing technologies should be improved. The literature is full of papers with thermo-optical switch (see, i.e., Maximizing the thermo-optic tuning range of silicon photonic structures. In 2007 Photonics in Switching (pp. 67-68). IEEE, (2007, August).; Design of a large bandwidth 2× 2 interferometric switching cell based on a sub-wavelength grating. Journal of Optics, 23(8), 085801(2021)). A comparison table should be added a table to compare SiN performances to other technological platforms in terms of dimension, cost, losses, thermos-optic coefficient, birefringent effect, refractive index contrast, coupling losses with SMF etc.
- In the Introduction Section, the Authors must add the references when make an assumption. (Line 51-55, line 64-66, line 68-70).
- In The Section 2 (Design and simulation), there is the major issue of the manuscript. All the design parameters lack of explanation. The choices of SiN, cladding and micro-heater thickness, width, the working wavelength are not justified.
- The refractive index of SiN has been evaluated by ellipsometer but the polymer one has not been explained. If it has been measured you have to clarify, if it has been found in literature you have to add a reference. Moreover, you must explicit what is the cladding polymer.
- You have to justify the choice of the tuning effect making a comparison (a table) among the thermo-optic, the electro-optic, the piezo-electric tuning and so on.
Although the Si3N4 platform ensures the lowest propagation losses at the state-of-the-art, any tuning approach leads an increase of the propagation losses (dB/cm) with a trade-off between losses and time response.
- You say that the gap has been set on 500μm to reduce thermal crosstalk but you have to insert a parametric study to evaluate how the crosstalk change with the gap size.
- Line 99, you say that taper waveguide are used. You have to explain why it is needed (for instance to make easier the coupling with fiber, to reduce the coupling loss etc.).
- Line 137, add the reference
- Line 147-148, you have to produce an image to show the thermo-optic coefficients evaluated for the polymer cladding and the SiN core and to add their values too.
- Line 159, explain the value of heat power density (50mW/mm). In addition, you said that the electrode length should be at least 419 μm so you have to clarify the reason why you set it on 1000 μm.
- The figure 2(a) should be improved.
- In table 2 you have to explain the value of heat power of electrodes (H1,H2,H3,H4)
- Line 192, you have to explain why you choose ZPU-12 as polymer making a comparison with other polymers in terms of refractive index and thermo-optic coefficient.
- You said that the characterization has been done as you have made before [ref 28, 29] but you do not say how it has been done by other authors.
- In table 3, you have to explain why H1 heat value is not considered. If it is not necessary you can remove it at all reducing the complexity of the structure. Moreover, the values of H2,H3,H4 are completely different from the previous one. You have to clarify.
- You have to discuss about the reliability of the structure, the total power consumption of the structure and the stability of the tuning mechanism. Furthermore, you said that the PECVD can be improved so I suggest you to do it and fix the results so that the justifications of the issues found can be improved.
- The TE mode has very high suppression, maybe you should try to improve the design parameters or the material used.
Author Response
Please see the attachment.
Author Response File: Author Response.docx
Round 2
Reviewer 2 Report
The Authors have modified the manuscript according to the Reviewer suggestions.
