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REVIEW 2 major objections 6 minor 57 references

Low-loss, fabrication-tolerant, and highly-tunable Sagnac loop reflectors and Fabry-P\'erot cavities on thin-film lithium niobate

T0 review · 2 major / 6 minor · reviewed 2026-08-07 · deepseek-v4-flash

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Pith's one-line read This paper reports a self-calibrated cavity measurement putting Mach-Zehnder interferometer loss on thin-film lithium niobate at 1.5% (-0.07 dB) per MZI, with a $2\times10^6$ intrinsic Fabry-P\'erot quality factor.

desk verdict Clever self-calibrated loss measurement and a genuine thermal-tuning advance are undermined by an internal inconsistency between the reported Qi and κi values that changes the per-MZI loss from 1.5% to 2.5%. read the letter →

arxiv 2505.23411 v1 pith:OKGEIMGQ submitted 2025-05-29 physics.optics physics.app-ph

Luke Qi , Ali Khalatpour , Jason Herrmann , Taewon Park , Devin Dean , Sam Robison , Alexander Hwang , Hubert Stokowski
show 3 more authors
Darwin Serkland Martin Fejer Amir H. Safavi-Naeini
This is my paper
classification physics.opticsphysics.app-ph PACS 42.82.-m42.60.Da
keywords thin-filmlithiumniobateSagnacloopreflectorMach-ZehnderinterferometerFabry-Pérotcavitythermo-opticphaseshifterthermalisolationtrenchqualityfactorintegratedphotonics
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

The authors set out to show that Mach-Zehnder interferometers on thin-film lithium niobate can be both low-loss and power-efficient, and to prove the loss with a method that does not depend on calibration of an external reference. They build each MZI into a Sagnac loop reflector, pair two such reflectors into a Fabry-P\'erot cavity, and read the MZI loss from the cavity's internal quality factor instead of from a single-pass transmission measurement. On their devices the intrinsic quality factor reaches $2\times10^6$, corresponding to a round-trip loss of 3% and, attributing that loss to the two MZIs, 1.5% ($-0.07$ dB) per MZI. Separately, substrate-undercut air trenches cut the thermo-optic $\pi$-shift power from about 80 mW to 2.5 mW. These results make it plausible to cascade many tunable MZIs on lithium niobate for large programmable photonic circuits.

What carries the argument

The load-bearing object is the MZI-controlled Sagnac loop reflector: an ordinary Mach-Zehnder interferometer with its two output ports joined by a loop, so that the whole structure acts as a two-port mirror. The reflected field is $E_r/E_{\mathrm{in}} = 2i\sqrt{\eta(1-\eta)}\sqrt{1-\gamma_{\mathrm{SLR}}}e^{i\phi_{\mathrm{SLR}}}$, and the scattering matrix contains the MZI matrix twice, $S_{\mathrm{SLR}} = S_{\mathrm{MZI}}^T S_x S_{\mathrm{MZI}}$; this double appearance is what cancels beamsplitter errors and makes the reflector work for coupling ratios from 15:85 to 85:15. Two SLRs are combined with a scattering-matrix star product into a Fabry-P\'erot cavity, and a symmetric double-sided bus-coupled Lorentzian fit separates the internal linewidth $\kappa_i$ from the external coupling $\kappa_e$; the round-trip loss follows from $l=2\pi f_0/(f_{\mathrm{FSR}}Q_i)$. Thermal efficiency comes from substrate-underetched air trenches that locally confine heat, raising the simulated waveguide temperature per milliwatt by about a factor of 25.

What would settle it

Build propagation-loss test resonators on the same chip and re-extract the per-MZI loss after subtracting their measured loss; if the inferred MZI loss changes by more than the fitting uncertainty, attributing the full round-trip loss to the MZIs is wrong, and driving the two mirrors with deliberately unequal heater powers would test whether the symmetric $\kappa_e$ assumption biases the internal linewidth.

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Extended reading notes

Core claim

This paper claims that an MZI whose two outputs are connected into a Sagnac loop is a mirror with reflectivity set by the MZI phase, and that the mirror is forgiving of imperfect beamsplitters because the light traverses the MZI twice. Two such tunable reflectors form a Fabry-P\'erot cavity; fitting the cavity modes with a symmetric double-sided bus-coupled model yields an internal quality factor $Q_i^{\mathrm{FP}}=2\times10^6$, from which the round-trip loss is $l=3\%$. Attributing that loss to the two MZIs in the cavity gives $1.5\%$ ($-0.07$ dB) per MZI, with a slightly lower $1.3\%$ if state-of-the-art propagation loss is subtracted first. The same chips show that substrate-underetched air trenches cut the thermo-optic $\pi$-shift power from about 80 mW to 2.5 mW, a more than tenfold improvement, at the price of reduced modulation bandwidth.

Load-bearing premise

The loss figure assumes the two Sagnac mirrors are identical because equal heater power is taken to mean equal external coupling, and assigns every unexplained cavity loss to the two MZIs; the post-undercut data show the mirrors responding asymmetrically, so the 1.5% per-MZI number could shift if that asymmetry affects the loss channels.

Editorial extensions

If this is right

  • Any MZI with beamsplitter coupling between 15:85 and 85:15 can still form a near-perfect Sagnac mirror, so the loss-measurement technique does not require precisely fabricated couplers.
  • The measured $Q_i^{\mathrm{FP}}=2\times10^6$ and 1.5% per-MZI loss mean the MZIs can be cascaded many times before loss dominates, supporting larger programmable photonic networks on TFLN.
  • The reflectors can be tuned to below -27 dB leakage and maintain high reflectivity over roughly a 1 THz window, limited by the dispersion of the directional couplers.
  • Substrate-undercut thermal isolation reduces the thermo-optic $\pi$-shift power to 2.5 mW from about 80 mW, enabling many actively tuned components on one chip, with a reduced modulation bandwidth as the trade-off.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • A cavity of the same type could characterize any two-port element that fits inside a Sagnac loop, not just an MZI—phase shifters, directional couplers, or modulators—on any waveguide platform, because the resonator isolates the element from input-coupling calibration.
  • The observed post-undercut offset $\phi_2 = 0.8\,\phi_1 + \pi/6$ suggests the suspended waveguides bow under local heating; fitting cavities with two independent external couplings, or tethering the waveguides, would test whether the symmetric model biases the extracted loss at higher finesse.
  • If the per-MZI loss survives separate propagation-loss subtraction, it would place TFLN meshes with hundreds of interferometers within reach for optical neural networks and quantum photonic circuits, where per-component loss currently caps system size.
  • The 25x simulated temperature gain from air trenches implies a clear power-bandwidth trade: designs that stagger the undercut windows or add tethers could recover modulation speed and mechanical stability while keeping most of the efficiency gain.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

2 major / 6 minor

Summary. The paper presents tunable Sagnac loop reflectors built from Mach-Zehnder interferometers on thin-film lithium niobate and uses Fabry-Pérot cavities formed by two such reflectors to extract MZI loss from the cavity quality factor. The central quantitative claim is a per-MZI insertion loss below 1.5% (-0.07 dB), obtained from Qi = 2×10^6 and fFSR = 19 GHz through the relation l = 2π f0/(fFSR Qi). The paper also reports Qe = 40×10^6, an internal loss rate 'around 150 MHz', and demonstrates thermal-isolation trenches that reduce the thermo-optic π-phase-shift power to 2.5 mW. A scattering-matrix model is used to reproduce tuning curves with an 80% beam-splitter coupling and, for the undercut devices, an empirical phase-offset relation between the two reflectors.

Significance. If the loss claim holds, the resonant self-calibrated approach is a valuable route to characterizing low-loss MZIs, and the demonstrated robustness of the Sagnac loop reflector to beamsplitter imbalance is practically important for programmable photonic circuits. The thermal-isolation result is straightforward and useful. The authors are to be credited for explicitly stating that the propagation loss was not measured and therefore the headline number is a conservative upper bound. However, the central quantitative claim is currently undermined by an internal inconsistency between the reported quality factor and the reported internal linewidth, so the manuscript must be revised before its significance can be fully assessed.

major comments (2)
  1. [Section III.B, Fig. 4d, and Section III.C] The reported internal quality factor Qi = 2.0×10^6 for the 1569 nm mode and the reported internal loss rate κi/2π ≈ 150 MHz for the same mode family are mutually inconsistent. For f0 ≈ 191 THz, Qi = 2×10^6 implies κi/2π = f0/Qi ≈ 95.5 MHz and, using the paper's formula l = 2π f0/(fFSR Qi) with fFSR = 19 GHz, a round-trip loss of about 3.2% (1.6% per MZI). In contrast, κi/2π = 150 MHz implies Qi ≈ 1.27×10^6 and a round-trip loss of about 5.0% (2.5% per MZI). These two reported values cannot both describe the same mode, and the headline '<1.5% per MZI' is supported only by the first value. The authors must re-extract Qi and κi from the raw spectra, provide uncertainty estimates, and report a consistent pair of values.
  2. [Section III.C and Section IV.B] The loss extraction relies on the symmetric double-sided bus-coupled cavity model with equal external coupling rates κe1 = κe2. The paper later shows that, after undercutting, the two reflectors are not symmetric and require the empirical relation φ2 = 0.8 φ1 + π/6 to reproduce the data. The authors should justify that the pre-undercut device used for the loss measurement is symmetric enough for the symmetric-fit assumption not to bias Qi, or they should quantify how the extracted κi changes if the two external couplings are allowed to differ.
minor comments (6)
  1. [Section III.B] Please provide the fit residuals and uncertainties for Qi and Qe; the current paper quotes single values without error bars, which is especially important given the discrepancy noted above.
  2. [Section III.C] The statement that 'the best we can claim is the conservative loss estimate of -0.07 dB per MZI' is a welcome caveat; please consider stating explicitly in the abstract that the '<1.5%' value is an upper bound that assumes all cavity loss is attributable to the two MZIs.
  3. [Section IV.B] The empirical relation φ2 = 0.8 φ1 + π/6 is introduced without detail about how it was determined; please state whether it is a fitted relation and show the sensitivity of the simulated tuning curves to the offset and slope.
  4. [Fig. 4b] It would be helpful to state how the 80% beam-splitter coupling ratio was obtained and whether this value is an independent measurement or a fit parameter, since the model validation depends on it.
  5. [General] The manuscript should clarify at which fabrication stage (before or after substrate undercut) the data in Figures 4c and 4d and the Qi = 2×10^6 measurement were taken; the text currently implies before undercut but does not state this explicitly.
  6. [Discussion] There is a typo in Section V: 'soley' should be 'solely'.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the per-MZI loss claim derives from a measured cavity Qi via a standard round-trip-loss relation, with an acknowledged literature-based propagation-loss subtraction.

full rationale

The central derivation chain is self-contained. The internal loss rate κi is obtained by fitting a double-sided bus-coupled Lorentzian to the measured reflection/transmission spectra (Fig. 4c,d and Eq. in Sec. III.C); the round-trip loss is then computed from the independently measured free spectral range and the fitted Qi using l = 2π f0/(fFSR Qi). The per-MZI attribution is explicitly stated as an assumption ('If we attribute all this loss due to the two MZIs within the Fabry–Pérot cavity'), and the propagation-loss subtraction uses external literature values (3 dB/m from refs. [18,34]) with the caveat that the device propagation loss was not measured. No fitted parameter is renamed as a prediction, and no load-bearing conclusion rests on a self-citation. The scattering-matrix 'simulation' of the tuning curves in Fig. 6 uses an 80% beamsplitter ratio taken from a separate control SLR and an explicitly acknowledged phase-offset fit (ϕ2 = 0.8 ϕ1 + π/6) to achieve qualitative agreement; because this is presented as a fit rather than as a prediction and does not feed into the loss extraction, it does not constitute circularity. A separate internal-consistency concern (Qi = 2×10^6 vs κi/2π ≈ 150 MHz) is a numerical/correctness issue, not a circularity of the derivation.

Assumptions & free parameters 2 free parameters · 5 assumptions · 0 invented entities

No new physical entities are postulated; the paper introduces a device structure (air-trench-isolated phase shifters) but not a new material, particle, or force.

free parameters (2)
  • MZI beamsplitter power coupling ratio = 80%
    Used in the SLR reflection simulation to match the measured tuning curve (Fig. 4b).
  • Reflector phase-offset relation = phi2 = 0.8 phi1 + pi/6
    Used in the post-undercut Fabry-Perot scattering simulation to reproduce the measured linewidth map (Fig. 6b).
assumptions (5)
  • domain assumption Beamsplitter, MZI, and Sagnac loop can be described by the scattering matrices in the paper (lossy and reciprocal).
    The loss model used to derive Eq. (1) and to fit the tuning curve assumes classical field propagation and independent loss factors.
  • domain assumption Equal heater power on both SLRs produces equal external coupling rates kappa_e1 = kappa_e2.
    Used to justify the symmetric double-sided cavity model in Section III.C and Eq. (12).
  • domain assumption Propagation loss of 3 dB/m from prior LNOI fabrication applies to the measured device.
    Used to subtract propagation contribution and estimate QiMZI=5e6; the authors explicitly state they did not measure their own propagation loss.
  • domain assumption Thermo-optic phase shift is proportional to electrical power and thermal conductivities from Table I are accurate.
    Used to interpret heater power as phase shift and to compare COMSOL simulations with measured tuning rates.
  • domain assumption Grating coupler wavelength response is smooth and does not distort the Lorentzian line shapes used for Q fitting.
    The reflected spectra show a background attributed to grating couplers; the fits subtract or ignore this background.

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Cite this review

Pith. "Pith review of Low-loss, fabrication-tolerant, and highly-tunable Sagnac loop reflectors and Fabry-P\'erot cavities on thin-film lithium niobate." pith.science (2026). https://pith.science/paper/OKGEIMGQ

@misc{pith2026250523411,
  author       = {Pith},
  title        = {Pith review of: Low-loss, fabrication-tolerant, and highly-tunable Sagnac loop reflectors and Fabry-P\'erot cavities on thin-film lithium niobate},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/OKGEIMGQ}},
  note         = {Machine review of arXiv:2505.23411}
}
abstract

We present low-loss ($<1.5\%$) and power-efficient Mach-Zehnder interferometers (MZIs) on thin-film lithium niobate. To accurately measure low MZI losses, we develop a self-calibrated method using tunable Sagnac loop reflectors (SLRs) to build cavities. Fabry-P\'erot cavities constructed from these fabrication-tolerant SLRs achieve an intrinsic quality factor of $2 \times 10^6$. By implementing thermal isolation trenches, we also demonstrate a $>10\times$ reduction in power consumption for thermo-optic phase shifters, achieving a $\pi$-phase shift ($P_\pi$) with just 2.5 mW. These tunable and efficient components are key for scaling up to complex photonic integrated circuits.

Figures

Figures reproduced from arXiv: 2505.23411 by the authors.

Figure 1
Figure 1. FIG. 1. Concept of tunable Sagnac loop reflectors with MZIs. a) Schematic of an SLR with a variable beamsplitter. Equivalent [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. FIG. 2. Simulated MZI outputs and SLR reflectivities. a) Bar output of a balanced MZI at various beamsplitter couplings [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. FIG. 3. Tunable Sagnac Fabry–P´erot cavities on LNOI. (a) [PITH_FULL_IMAGE:figures/full_fig_p003_3.png] view at source ↗
Figures from the paper (8 more)
Figure 4
Figure 4. Figure 4: FIG. 4. Characterization of tunable SLRs and Fabry–P´erot cavities. a) Measurement setup. b) Tuning curve of SLR and [PITH_FULL_IMAGE:figures/full_fig_p004_4.png]
Figure 5
Figure 5. Figure 5: FIG. 5. TOPS characterization with (a) normal heater (b) [PITH_FULL_IMAGE:figures/full_fig_p006_5.png]
Figure 6
Figure 6. Figure 6: The labeled “highly reflective” region leads to [PITH_FULL_IMAGE:figures/full_fig_p006_6.png]
Figure 6
Figure 6. Figure 6: FIG. 6. Full mode tuning of the Fabry–P´erot resonators (a) before undercut (b) after undercut. Top left: measured experimental [PITH_FULL_IMAGE:figures/full_fig_p007_6.png]
Figure 7
Figure 7. Figure 7: FIG. 7. Model methodology. (a) Beamsplitter 2x2 element. (b) MZI as a cascade of beamsplitters and phase shifters. (c) MZI [PITH_FULL_IMAGE:figures/full_fig_p011_7.png]
Figure 8
Figure 8. Figure 8: FIG. 8. Fabry-P´erot modeling. (a) Equivalent pictures between mirrors and Sagnac loop reflectors. (b) Diagram of feedback [PITH_FULL_IMAGE:figures/full_fig_p012_8.png]
Figure 9
Figure 9. Figure 9: FIG. 9. MZI outputs with any beamsplitter ratios. (a) Maximum achievable bar output of the MZI. (b) Maximum achievable [PITH_FULL_IMAGE:figures/full_fig_p014_9.png]
Figure 10
Figure 10. Figure 10: FIG. 10. Device photos and data. (a) EvoCam image of MZI-controlled SLR Fabry-Per´ot cavity. (b) iPhone photograph of [PITH_FULL_IMAGE:figures/full_fig_p014_10.png]

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