Compact and high-resolution spectrometer via Brillouin integrated circuits

Domain: physics
Published: 2025-11-29
Editorial outcome: minor_revision

Abstract
Optical spectrometers are indispensable tools across various fields, from chemical and biological sensing to astronomical observations and quantum technologies. However, the integration of spectrometers onto photonic chips has been hindered by the low spectral resolution or large device footprint with complex multiple channel operations. Here, we introduce a chip-integrated spectrometer by leveraging the acoustically-stimulated Brillouin scattering in a hybrid photonic-phononic chip. The Brillouin interaction provides a dynamic reflection grating with a high reflectivity up to 50% and a fast switching time on the microsecond scale, achieving a spectral resolution of 0.56 nm over a 110 nm bandwidth using just a single 1 mm-long straight waveguide. This performance approaches the fundamental limit of resolution for a given device size, validating the potential of the hybrid photonic-phononic device for efficient and dynamically-reconfigurable spectral analysis, and thus opens up the avenues for advanced optical signal processing and sensing applications.

Introduction
Photonic integrated circuits (PICs) have emerged as a powerful platform1–6, which has the potential to revolutionize the way we generate, process, and manipulate light. By integrating various optical components onto a single chip, PICs enable the realization of sophisticated optical systems that offer unparalleled scalability, low power consumption, high stability, and enhanced light-matter interaction. In recent years, significant progress has been made in the development of PIC technologies, leading to notable achievements in various fields, such as ultra-high data transmission rates in optical communications7, ultra-sensitive biosensors8, and quantum photonic devices for secure communication and computation9. To further unlock the capability of PIC in spectroscopic analysis for astronomy measurements10,11, medical diagnostics12, environmental monitoring13, and chemical analysis14,15, high-performance optical spectrometers are indispensable16–18.

However, the full integration of spectrometers onto photonic chips faces a fundamental trade-off between spectral resolving capability and device footprint16. Conventional spectrometers mostly rely on static dispersive elements, which could be directly extended to PICs19–21. For example, the grating shown in Fig. 1a separates light of different colors into distinct channels. While the grating offers a wide detection wavelength range, achieving high spectral resolution necessitates a large device length and a dense array of collection channels. The spectral resolution of a grating-based spectrometer is approximately given by Δ λ=λ2=n_eff L sinθ_1 + sinθ_2, where L is the grating length, n_eff is the effective refractive index of the guided optical mode, λ is the light wavelength, and θ is the angle between the input (output) optical beam and the grating. An alternative approach is to employ dynamically tunable dispersive devices, such as Mach-Zehnder interferometers22–24 shown in Fig. 1b. These devices offer potential advantages including higher optical throughput, reduced device footprint, and requirement of fewer detection channels. Nonetheless, the resolution of this approach is limited by the device’s tuning capability. The spectral resolution of a tunable interferometer-based spectrometer is given by Δ λ=λ2/n_eff ΔL with ΔL is the tuning path difference of the interferometer. The requirement for a large tuning length δnΔL ≫ λ, where δn is the variation of the material refractive index. This requirement imposes a practical limitation of the spectrometer size L, limiting it to the order of centimeters, even with the enhancement provided by microrings25. Consequently, there is a pressing need for an approach that combines the advantages of both static and dynamic dispersive elements, enabling the realization of compact, single-channel on-chip spectrometers.

Here, we introduce a fundamentally different approach to integrated spectroscopy that leverages acoustically-stimulated backward Brillouin scattering, a nonlinear optical interaction between photons and acoustic phonons in a medium (Fig. 1c). By harnessing the strong interaction between guided optical and acoustic modes in a carefully engineered lithium niobate (LN) waveguide, we demonstrate a high-resolution, compact, and fully integrated spectrometer with an intrinsic spectral resolution of 0.56 nm over a wavelength range exceeding 110 nm, while maintaining a compact footprint of just 1 mm in length. Our approach provides a coherent interface between the optical spectrometer and radio frequency (RF) control, allowing fast spectral sweeping, with a typical sweep duration of 0.1 seconds and could potentially be even faster. Furthermore, our demonstration of a hybrid photonic-phononic circuit can be extended to incorporate other photonic, electronic, and phononic devices on a single chip, paving the way for fully integrated and multi-functional spectroscopic systems and also for a wide range of photonic-phononic hybrid devices.

Results
Figure 1 illustrates the conceptual schematics of conventional integrated spectrometers and the principle of the proposed approach based on Brillouin scattering9,26–29. Traditional on-chip spectrometers typically rely on linear optical processes, such as dispersive elements in Fig. 1a or Fourier-transform techniques in Fig. 1b. In contrast, our spectrometer exploits the nonlinear Brillouin interaction between guided photonic and phononic modes in a compact waveguide platform (Fig. 1c)30. At the heart of our device is a straight waveguide that supports both optical and acoustic modes, which are coupled primarily through the photoelastic effect. The waveguide is integrated with a compact interdigital transducer (IDT) for RF-to-acoustic wave conversion and with grating couplers that enable efficient excitation and detection of the optical response. When operating the spectrometer, an input optical signal (ω_1) is coupled into the waveguide and interacts with a counter-propagating acoustic wave (Ω) generated by the IDT.

The Brillouin scattering process manifests as a coherent three-wave mixing (TWM) interaction, leading to a wavelength-dependent coupling between forward and backward optical signals stimulated by acoustic wave fields. This coherent TWM results in a frequency-shifted output signal (ω_2) that satisfies both the energy and momentum conservation conditions: ω_2 = ω_1 + Ω and k_2 = k_1 + β, where k and β are the wave vectors of the input (output) optical signal and acoustic wave, respectively. In a simpler picture, the acoustic wave modulates the refractive index along the waveguide through the photoelastic effect, and thus the waveguide can be understood as a dynamic optical grating. Therefore, our spectrometer resembles a linear grating spectrometer (Fig. 1a) with the corresponding input and output angles θ_1 = θ_2 = π/2, allowing us to achieve a spectral resolution approaching the theoretical upper bound of λ2/2n_eff L for a given interaction length L. Moreover, our single-channel device can resolve a wide spectral range through dynamically reconfigurable β by simply sweeping the external RF drive frequency.

To experimentally validate our ultra-compact spectrometer, we fabricate the hybrid phononic and photonic circuits using thin-film LN on sapphire substrate. The combination single-crystal LN and sapphire allows for low-loss photonic and phononic waveguides, while LN’s strong piezoelectric and photoelastic properties facilitate efficient interfacing among the RF, acoustic, and optical signals, making it an ideal material platform for our spectrometer. The device, shown in Fig. 2b, features a straight waveguide with a width of 1.2 μm, a thickness of 400 nm, and a wedge step of 180 nm. This carefully designed geometry supports both optical modes at telecom wavelengths and acoustic modes around 8.5 GHz, with a phase-matched phonon wavelength of 400 nm. The corresponding mode profiles of the photonic and phononic modes are shown in Fig. 2d and g. To excite and focus the acoustic wave into the waveguide, we employ a fan-shaped IDT (Fig. 2c) driven by a RF signal from a vector network analyzer (VNA). The RF S11 parameter of the transducer, as shown in Fig. 2f, exhibits a resonance frequency of 8.5 GHz, indicating the efficient generation of high-frequency guided phonons into the waveguide.

To efficiently couple phonons and photons into the same waveguide for Brillouin interaction, while protecting the electrodes from potential damage caused by direct interaction between the input laser and the IDT, we have designed a hybrid phononic-photonic multiplexing device, as shown in Fig. 2e. This device exploits the tighter confinement of the phononic mode, allowing it to propagate through the waveguide with minimal coupling to adjacent waveguide due to the weak evanescent acoustic field. In contrast, the photonic mode can be efficiently transferred between the two waveguides when passing through the coupling region. Optical input and output couplings are achieved using grating couplers for high signal-to-noise ratio or using side couplings to avoid the sensitivity of coupling efficiency to wavelength and achieve a large wavelength bandwidth couple (A more detailed discussion provided in the Supplementary Information II.)

The experimental setup for characterizing the Brillouin scattering is depicted in Fig. 2a. A tunable laser, amplified by an erbium-doped fiber amplifier (EDFA) and with its polarization controlled, is coupled into the waveguide. The reflected optical signal at the selected wavelength, which satisfies the phase-matching condition of Brillouin scattering in the spectrometer, is collected and directed to a photodetector. To investigate the spectrometer’s performance, we fixed the input optical wavelength at 1576 nm and scanned the input RF signal supplied by the VNA, measuring the reflected light intensity with an interaction length L = 1 mm. The result, shown in Fig. 2h, demonstrates the remarkably reflection of the acoustically-induced dynamic grating. The spectrum exhibits a narrow linewidth, with a full width at half maximum (FWHM) of just 3.4 MHz, validating the highly selective spectral response and also sub-μs switching time for high-resolution on-chip spectral analysis. Moreover, with an RF drive power of only 20 mW, as shown in Fig. 3a, the grating achieves a peak reflectivity of 50%, which highlights the efficient Brillouin interaction enabled by the carefully engineered hybrid photonic-phononic waveguide.

Figure 3 demonstrates the broadband performance of our spectrometer. In Fig. 3a, the responses for monochromatic optical input signals are measured for different wavelengths ranging from 1514 to 1624 nm, with an experimental wavelength of 110 nm limited by the spectral range of our testing optical source. The linear dependence between the input and reflected optical powers, as shown in the left inset of Fig. 3a, verifies the coherent TWM process of acoustically-stimulated Brillouin scattering. The right inset of Fig. 3a depicts the linear dispersion relationship between the frequency shift of the output light and the input optical wavelength, with a slope of 6.03 MHz/nm. Combined with the narrow bandwidth of the RF response [Fig. 2h], we derive an optical intrinsic spectral resolution of 0.56 nm. The envelope of the varying peak values represent the frequency-dependent RF-phonon conversion efficiency, which is determined by the bandwidth of the IDT. It is worth noting that the observed fluctuations on this envelope reflect the complete, wavelength-dependent system transfer function, which includes not only the IDT response but also factors such as the intrinsic acousto-optic coupling strength and on-chip polarization effects. This entire repeatable response is captured in the calibration matrix used for spectral reconstruction. Although the experimental data is acquired over a 110 nm range, our analysis (Supplementary Information) indicates that the spectrometer can operate over a significantly broader wavelength range. Based on the characteristics of the current IDT design, we estimate that the dynamic range of the spectrometer exceeds 150 nm. Furthermore, by employing advanced broadband IDT designs or cascaded IDTs on a single waveguide, the operating range can potentially be extended to 300 nm or more.

To directly demonstrate the sub-nanometer resolution of our spectrometer, we perform a spectral measurement using a source consisting of two laser tones as input, with a wavelength interval of only 0.5 nm. By scanning the RF drive frequency, we obtain the spectral information shown in Fig. 3b. The raw data clearly show two distinctly resolved peaks. We fit the data using a double-Lorentzian function, which is the sum of two Lorentzian distributions displayed in red. Furthermore, by using the spectral response for monochromatic light in Fig. 3a, we reconstruct the input spectrum with respect to the optical wavelength. The retrieved wavelength data are consistent with our input laser wavelengths, proving that the projected resolution of our spectrometer can be much better than 0.5 nm.

To further demonstrate the fluorescence measurement capability of our spectrometer and its potential in telecom applications, we use an EDFA to generate a broadband spectrum and utilize a dense wavelength division multiplexing (DWDM) filter to synthesize a broadband light source. The DWDM filter allows us to selectively switch on and off specific bandpass channels (e.g., C23-C26) to create controlled spectral features. As shown in Fig. 3c, the spectrum is first measured using a commercial spectrometer as a reference. In our device, the input signal from our device is directly detected by a PD. The response spectrum in Fig. 3c (blue line) reveals a frequency-independent background noise floor, which we attribute to reflections of the input signal in our system, primarily due to the backward scattering from the coupler. By mapping the RF response to the optical domain, we derive the optical spectrum, which exhibit excellent agreement with the reference spectrum measured by a commercial spectrometer (orange line). This measurement result demonstrates that our spectrometer possesses the capability to resolve fluorescence spectra and is well-suited for DWDM, which is widely used in the telecom technology.

In Fig. 4a, we compare the performance of our device with state-of-the-art demonstrations reported in the literature19–23,25,31–35, focusing on the intrinsic spectral resolution of the devices. The intrinsic spectral resolution is governed by the dispersive path of the device, which is determined by its inherent optical response bandwidth, regardless of the input signal strength, detection noises, and the reconstruction algorithms36–40. Our device exhibits high performance in terms of resolution and footprint. According to the coherent mode conversion in an ideal uniform waveguide, the intrinsic resolution is determined by the phase-matching condition in the coupled mode equations41 (Supplementary Information). For a fixed RF frequency, the reflected light has a bandwidth corresponding to the FWHM of the wave vector Δ β ≈ 5.56/L, which implies a fundamental trade-off relation between resolution and footprint. Considering the dispersion relation of the photonic mode as ∂β/∂λ = 1.2 × 10^4 rad/(m · nm), the fundamental limitation of resolution is given by Δ λ = Δ β / (∂β/∂λ) = 460 / L μm nm: (1)

For an interaction length L = 1 mm in our device, the resolution limit for an ideal lossless waveguide is 0.46 nm, indicating that our experimental result of 0.56 nm approaches this fundamental limit with a small discrepancy attributed to the losses. Also, we find that the practical resolution of our spectrometer does not improve with further increases in L, revealing the constraints imposed by the propagation loss of the phononic mode in the waveguide. As shown by the red lines in Fig. 4b, the resolution considering the phonon attenuation is calculated for given equivalent phononic quality factors (Q), and a phonon Q ~ 3000 explains our observations of the saturated resolution for L > 1 mm. Furthermore, the potential imperfections of non-uniform waveguide cross-section can introduce variations in the mode wave vectors, which can be treated as a phase-matching diffusion process in the mode conversion and reduce the resolution. The dependence of the phase mismatching Δβ on the variation of the waveguide width (Δw) and film thickness (Δt) is shown in Fig. 4c, demonstrating that the two geometry parameters independently contribute to the mismatching. By assuming a random walk of Δβ and the accumulated mismatch in the interaction region following a normal distribution, we numerically simulate the influence of the phase-matching diffusion with a standard deviation σ = 1500 rad/m for different phonon Q (Supplementary Information). The results are plotted in Fig. 4b as dots, with the shaded area representing the standard deviation. These results show that the impact of phase-matching diffusion becomes more pronounced for higher Q. Therefore, a resolution of 0.1 nm with an interaction length under 6 mm is feasible by improving phonon Q by one order of magnitude42 and suppressing the variation of Δβ through compensating for film thickness variations via specially designed modulations in the waveguide width43.

Methods
Device fabrication process
We fabricate the hybrid phononic and photonic circuits using LN-on-sapphire structure. In this work, we use x-cut LN-on-sapphire wafer featuring 400 nm LN layer on a 400 μm thick sapphire substrate. The fabrication process of the LN-on-sapphire chip involves two main steps. First, we define the optical waveguides using hydrogen silsesquioxane (HSQ) as the resist using electron beam lithography (EBL). The structures are then etched using argon etching. The optical wedge waveguides are formed by partially etching the LN layer by 220 nm, leaving a 180 nm-thick slab layer. Second, an interdigital transducer (IDT) structure are defined in another EBL step using polymethyl methacrylate (PMMA) resist. A layer of 20 nm Aluminum is then deposited on the top of LN using an electron beam evaporator, followed by a liftoff process.

Spectrometer characterizing
In our experiment, we designed different optical coupling structures and testing setups for various application scenarios. Regarding optical coupling, we utilized different coupling methods. We employed both grating coupling and side coupling to achieve efficient input-output relationships. Meanwhile, we implemented two distinct testing methods: one is the heterodyne detection method mentioned in Fig. 2(a), and the other directly measures the intensity of the reflected signal via a photodiode shown in Fig. 3c. Side coupling is typically combined with heterodyne detection due to its advantage in enabling broadband signal measurement without the coupling efficiency being affected by wavelength. However, side coupling tends to introduce significant background reflection noise, which can lower the signal-to-noise ratio (SNR) when directly reading the reflected signal power. To address this, heterodyne detection is employed to enhance the SNR, as demonstrated by the data in Fig. 3a.

Data availability
All data generated or analysed during this study are available within the paper and its Supplementary Information. Further source data will be made available on request.

Code availability
The code used to solve the equations presented in the Supplementary Information will be made available on request.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript describes a novel on-chip high resolution spectrometer\nbased on Brillouin scattering.\nThe device employs a lithium niobate thin film rib waveguide on a\nsapphire substrate.\nRather than a stimulated Brillouin approach, the GHz acoustic wave\nis generated by an on-chip IDT device directed into the waveguide.\nBy backward Brillouin scattering, the incoming light is reflected\nand Stokes shifted when Brillouin-resonant with the incoming acoustic\nwave. By scanning the IDT driving frequency and identifying\nthe frequency at which strong reflection is obtained, the resonant frequency\ncan be identified with precision of about 3.4 MHz.\nThe authors show this provides the ability to distinguish optical signals\nseparated by approximately 0.5 nm which is comparable to many commercial\nspectrometers. The device has an operating range of approximately 100 nm.\nThe authors also perform an interesting analysis of the resolution performance\nof their device based on phase diffusion of the interaction due to random\nvariations in the device dimensions along its length and show the capacity\nfor some improvement if both elastic losses and device variations can be better\ncontrolled.\nAs far as I can tell, this is the first demonstration of IDT-induced\non-chip Brillouin scattering for spectroscopy.\nThe final performance chart comparing the footprint and resolution of this\ndevice with existing literature is very impressive and would suggest\nthat this technology could have significant commercial impact for\ncompact precision optical spectroscopy.\nOn those bases, I believe the paper could warrant publication in Nature Communications.\nThe paper is well written with excellent figures and a useful supplementary material section.\nA significant issue which would need to be remedied is that it does not appear to\ncite any of the recent papers on Brillouin scattering in thin film lithium niobate,\nnotably those from the Marpaung and Wiederhecker groups. Their works have focused\non stimulated Brillouin scattering rather than IDT-induced elastic waves,\nbut they have provided the basic understanding of surface mode elatic wave propagation\nwhich makes Brillouin scattering possible in this material system.\nFigure 2 makes reference to the 'quasi-Love' mode but without citation or any further\nexplanation. Indeed there is no mention in the text of how the numerical modelling\nof optical and elastic waves was performed. Was this done using Comsol,\nanother well-known tool or home-grown software?\nI also wondered about the suggestion of a potential elastic Q of 20000.\nWhat is the upper limit to this imposed by viscosity?", "reviewer_2": "Journal: Nat Comm\nManuscript ID: NCOMMS-25-52018-T\nTitle: Compact and high-resolution spectrometer via Brillouin integrated circuits\nThis paper presents a chip-integrated spectrometer by leveraging acoustically-stimulated Brillouin scattering in a hybrid\nphotonic-phononic chip. For the phonon generation, the authors utilised external IDT on LiNbO3 at around 8.5GHz,\nmatching to the Brillouin shift of the waveguide. The Brillouin interaction provides a dynamic reflection grating and a fast\nswitching time on the microsecond scale, achieving a spectral resolution of 0.56 nm over a 110 nm bandwidth using 1 mm-\nlong straight lithium niobate waveguide. The principle of spectrometer is rather simple, coming from the linear relationship\nbetween Brillouin shifts and pump wavelengths – and more importantly very narrow Brillouin gain bandwidth (3.4MHz) \nwhich allows high resolution.\nOverall, the manuscript is well-organised and written. And I presume the paper could be quite interesting to relevant\ncommunity.\nHere are my comments.\n- One core performance of the device is very narrow Brillouin gain bandwidth (3.4MHz). However, there is no clear\nexplanation on this. Why is the phonon lifetime so long in this chip? Is it related to the material properties of LN and\nsapphire?\n- The authors also suggested the acoustic Q factor could be increased by 7-folds, but there is no detail on how to. They just\nadded a reference 38 which is on bulk LN (not thin film LN) – I could not find any clue how to improve phonon-Q.\n- The platform is Zhengfu architecture – LN on Sapphire. The authors did not explain why they use this specific combination.\nOne trivial question would be whether conventional thin film LN on SiO2/Si can be used for this application. If it works, the\nauthors explain why they use this architecture. If not, the authors explain why the conventional platform is not working.\n- It is clear the device can measure the spectrum of input lights. However, I am not convinced on the accuracy of light\nintensity measurement. At a single wavelength, the reflected optical power has a linear relation (in log-scale) with input\npower. The linear coefficients at different wavelengths are the function of the frequency-dependent RF-phonon conversion\nefficiency, which is determined by the bandwidth of the IDT (yellow shaded area in Fig3-a). However, I found that the\nreflected light powers are not following the conversion efficiency curve. In this situation, it would be very difficult to guess the\npower of input light at different wavelengths.", "reviewer_3": "The manuscript titled “Compact and high-resolution spectrometer via Brillouin integrated circuits” demonstrates a\nwaveguide-based device integrated with interdigital transducer to excite acoustic waves which then scatter the input optical\nsignal. The study proposes its use as a compact spectrometer.\nI do not see it any different from a pump-probe Brillouin scattering experiment in an integrated waveguide, where tuning the\npump wavelength results in tuning of the Stokes frequency at which maximum gain would occur. Further, the input power of\nthe optical signal has to be above a few hundreds microwatts for the device to generate a detectable backscattered signal\nwith good signal-to-noise ratio, which places a minimum threshold on the power of the optical signal to be detected. Further,\nit will be better to compare the input spectrum using the both commercial spectrometer and the proposed device using the\nsame scale (log or linear).\nThe resolution of 0.56 nm achieved in this demonstration is much lower than what has been obtained using the Brillouin\ndynamic gratings in optical fiber (Optics Letters Vol. 39, Issue 10, pp. 2967-2970 (2014)) or photonic chip (Optics Letters Vol.\n38, Issue 3, pp. 305-307 (2013))."}, "author_response": "List of Changes:\n1. We have added three additional references (Nos. 27–29) associated with integrated\nlithium niobate Brillouin scattering.\n2. We have added the No. 30 reference on integrated phononic waveguide modes.\n3. In the caption of Figure 2, we added a sentence stating, “All these simulations were\nperformed using the finite element method (COMSOL Multiphysics)” to clarify our\nnumerical simulation method for phononic modes.\n4. In Section VI of the Supplementary Information, we have added two paragraphs to\ndiscuss how to further enhance the acoustic Q-factor: “In addition to expanding the\nworking bandwidth, further improvements lie in enhancing the intrinsic resolution of\nthe spectrometer by increasing the phonon quality factor. Current devices exhibit a Q-\nfactor of approximately 3000. We believe this value is not constrained by the intrinsic\nacoustic properties of single-crystal lithium niobate but rather by external loss\nmechanisms. The significant difference between the experimentally measured quality\nfactor and the higher Q value derived from bulk material studies indicates substantial\nroom for improvement. These external losses primarily arise from two sources:\ninherent defects in the thin-film wafers and defects introduced during the\nnanofabrication process.\nTherefore, future performance enhancements can be realized through two approaches:\nsystematically optimizing nanofabrication processes to reduce surface scattering\nlosses, and further improving the thin-film material preparation techniques, such as\nenhancing wafer bonding processes and developing defect repair or annealing\ntechnologies suitable for this material system. Successfully addressing these\nchallenges will pave the way for the emergence of a new generation of ultra-high-\nresolution chip-scale Brillouin spectrometers.”\n5. We have added a sentence in the main text in page 5 to explain the different between\nthe measured reflection peaks in Figure 3a and the idealized IDT efficiency envelope:\n“ It is worth noting that the observed fluctuations on this envelope reflect the complete,\nwavelength-dependent system transfer function, which includes not only the IDT\nresponse but also factors such as the intrinsic acousto-optic coupling strength and on-\nchip polarization effects. This entire repeatable response is captured in the calibration\nmatrix used for spectral reconstruction.”\n6. We have updated Figure 3c, which plot the data from both our device and the\ncommercial spectrometer on a normalized linear scale to better illustrate the high\nfidelity of our instrument.\nAdditionally, we have updated the caption: “The blue line represents the reflection\npower from our device via sweeping microwave freqeuncy and detected by a PD\ndirectly, and the orange line comes from a commercial spectrometer.”\nResponse to Reviewer #1\nGeneral comment: This manuscript describes a novel on-chip high resolution spectrometer\nbased on Brillouin scattering. The device employs a lithium niobate thin film rib waveguide on\na sapphire substrate. Rather than a stimulated Brillouin approach, the GHz acoustic wave is\ngenerated by an on-chip IDT device directed into the waveguide. By backward Brillouin\nscattering, the incoming light is reflected and Stokes shifted when Brillouin-resonant with the\nincoming acoustic wave. By scanning the IDT driving frequency and identifying the frequency\nat which strong reflection is obtained, the resonant frequency can be identified with precision\nof about 3.4 MHz. The authors show this provides the ability to distinguish optical signals\nseparated by approximately 0.5 nm which is comparable to many commercial spectrometers.\nThe device has an operating range of approximately 100 nm.\nThe authors also perform an interesting analysis of the resolution performance of their device\nbased on phase diffusion of the interaction due to random variations in the device dimensions\nalong its length and show the capacity for some improvement if both elastic losses and device\nvariations can be better controlled.\nAs far as I can tell, this is the first demonstration of IDT-induced on-chip Brillouin scattering\nfor spectroscopy. The final performance chart comparing the footprint and resolution of this\ndevice with existing literature is very impressive and would suggest that this technology could\nhave significant commercial impact for compact precision optical spectroscopy. On those bases,\nI believe the paper could warrant publication in Nature Communications.\nThe paper is well written with excellent figures and a useful supplementary material section.\nResponse: We thank Reviewer #1 for the thorough and insightful review of our paper. The\nReviewer's recognition of our work as “first demonstration”, “very impressive”, “significant\nimpact”, and “warrant publication in Nature Communications” is particularly encouraging. We\nare also very grateful for the constructive suggestions, which have helped us significantly\nimprove this paper.\nIn the following, we will address the Reviewer's comments one by one.\nComment 1: A significant issue which would need to be remedied is that it does not appear to\ncite any of the recent papers on Brillouin scattering in thin film lithium niobate, notably those\nfrom the Marpaung and Wiederhecker groups. Their works have focused on stimulated\nBrillouin scattering rather than IDT-induced elastic waves, but they have provided the basic\nunderstanding of surface mode elatic wave propagation which makes Brillouin scattering\npossible in this material system.\nResponse: We thank the Reviewer for raising this comment, and we apologize for this oversight.\nIn the revised manuscript, we have incorporated the relevant work of the two related research\ngroups into the first paragraph of the Results and numbered as 27-29 :\n[27] K. Ye, H. Feng, R. Te Morsche, C. Wei, Y. Klaver, A. Mishra,Z. Zheng, A. Keloth, A. Tarık\nIsık, Z. Chen, et al., “Integrated brillouin photonics in thin-film lithium niobate,” Science Ad-\nvances 11, eadv4022 (2025).\n[28] C. C. Rodrigues, N. J. Schilder, R. O. Zurita, L. S. Magalhaes, A. Shams-Ansari, F. J. Dos\nSantos, O. M. Paiano, T. P. Alegre, M. Loncar, and G. S. Wiederhecker, “Cross-polarized\nstimulated brillouin scattering in lithium niobate waveguides,” Physical review letters 134,\n113601 (2025).\n[29] G. S. Wiederhecker, P. Dainese, and T. P. Mayer Alegre, “Brillouin optomechanics in\nnanophotonic structures,” APL photonics 4 (2019), 10.1063/1.5088169.\nComment 2: Figure 2 makes reference to the 'quasi-Love' mode but without citation or any\nfurther explanation. Indeed there is no mention in the text of how the numerical modelling of\noptical and elastic waves was performed. Was this done using Comsol, another well-known\ntool or home-grown software?.\nResponse: Thank you for your question. The term 'quasi-Love' mode, mentioned in the caption\nof Figure 2, describes the specific acoustic modes in our waveguide. A classical Love wave is\na guided acoustic wave where the particle motion is purely an in-plane (horizontal) shear\ndisplacement. In our acoustic waveguide, the 'quasi-Love' mode is an acoustic mode\ncharacterized by predominantly horizontal vibration. Further, this 'quasi-Love' mode is ideal\nfor our application because its strong in-plane shear component couples very efficiently to the\noptical TE mode via lithium niobate's photoelastic tensor, and is easy to excite via IDT, leading\nto the strong Brillouin interaction we utilize. For a more comprehensive theoretical treatment\nof such guided acoustic waves in thin films, we refer to Wang et al. (W. Wang, M. Shen, C.-L.\nZou, W. Fu, Z. Shen, and H. X. Tang, High-Acoustic-Index-Contrast Phononic Circuits:\nNumerical Modeling, J. Appl. Phys. 128, (2020)), which we now cite as Ref [30] in the main\ntext.\nRegarding the numerical modeling, all numerical results of the optical and acoustic modes, such\nas the mode profiles shown in Figures 2c and 2g, were performed using the finite element\nmethod with the commercial software COMSOL Multiphysics. To clarify this point, we have\nadded a sentence in the caption of Fig. 2 as:\n“Displacement and electric field distributions at the waveguide cross-section for the quasi-\nLove phononic mode and the fundamental transverse electric photonic mode, respectively.\nAll these simulations were performed using the finite element method (COMSOL\nMultiphysics).”\nComment 3: I also wondered about the suggestion of a potential elastic Q of 20000. What is\nthe upper limit to this imposed by viscosity?\nResponse: We appreciate the reviewer's insightful comment. The potential acoustic Q-factor\nof 20,000 proposed in our paper is based on the experimental results reported in Ref. [42] (I. L.\nBajak, A. McNab, J. Richter, and C. D. W. Wilkinson, Attenuation of Acoustic Waves in Lithium\nNiobate, J. Acoust. Soc. Am. 69, 689 (1981)). According to the measurement in bulk lithium\nniobate, we estimate the corresponding quality factor for bulk lithium niobate at a frequency of\n8.5 GHz. This value represents a lower bound of the achievable acoustic quality factor that\nlimited by the material absorption.\nThe discrepancy between this potential value and the Q-factor of ~3000 measured in our device\nstrongly indicates that our current performance is limited by extrinsic loss mechanisms\nintroduced during fabrication, rather than by the material absorption itself. These extrinsic\nlosses arise from two primary sources:\n1) The imperfections arising from the wafer production process. The commercial LN-on-\nsapphire films are typically produced via an ion-slicing method, which is known to\nintroduce residual lattice damage that acts as a scattering center for phonons.\n2) The surface scattering from the etched imperfect sidewalls of the waveguide. Because our\nphononic mode is tightly confined, it interacts strongly with these surfaces, meaning any\nfabrication-induced roughness causes significant phononic scattering loss.\nConsidering these factors, performance improvements can be achieved by optimizing\nnanofabrication processes to minimize the surface scattering-related losses. Furthermore, to\nreach the ultimate quality factor constrained by the intrinsic properties of the materials, future\ndevelopments in thin-film material preparation are needed, such as advancements in wafer\nbonding techniques and defect repair technologies. We add a paragraph in Supplement VI part\nto discussing this as following:\n“In addition to expanding the working bandwidth, further improvements lie in enhancing the\nintrinsic resolution of the spectrometer by increasing the phonon quality factor. Current devices\nexhibit a Q-factor of approximately 3000. We believe this value is not constrained by the\nintrinsic acoustic properties of single-crystal lithium niobate but rather by external loss\nmechanisms. The significant difference between the experimentally measured quality factor and\nthe higher Q value derived from bulk material studies indicates substantial room for\nimprovement. These external losses primarily arise from two sources: inherent defects in the\nthin-film wafers and defects introduced during the nanofabrication process.\nTherefore, future performance enhancements can be realized through two approaches:\nsystematically optimizing nanofabrication processes to reduce surface scattering losses, and\nfurther improving the thin-film material preparation techniques, such as enhancing wafer\nbonding processes and developing defect repair or annealing technologies suitable for this\nmaterial system. Successfully addressing these challenges will pave the way for the emergence\nof a new generation of ultra-high-resolution chip-scale Brillouin spectrometers.”\nResponse to Reviewer #2\nGeneral comment: This paper presents a chip-integrated spectrometer by leveraging\nacoustically-stimulated Brillouin scattering in a hybrid photonic-phononic chip. For the\nphonon generation, the authors utilised external IDT on LiNbO3 at around 8.5GHz, matching\nto the Brillouin shift of the waveguide. The Brillouin interaction provides a dynamic reflection\ngrating and a fast switching time on the microsecond scale, achieving a spectral resolution of\n0.56 nm over a 110 nm bandwidth using 1 mm-long straight lithium niobate waveguide. The\nprinciple of spectrometer is rather simple, coming from the linear relationship between\nBrillouin shifts and pump wavelengths – and more importantly very narrow Brillouin gain\nbandwidth (3.4MHz) which allows high resolution. Overall, the manuscript is well-organized\nand written. And I presume the paper could be quite interesting to relevant community.\nResponse: We thank Reviewer #2 for the careful and thorough reading of our manuscript and\nthe evaluation of our work as “quite interesting to relevant community”. We also appreciate the\nReviewer for raising valuable comments for the purpose of clarifying and improving our\nmanuscript.\nIn the following, we address the comments raised by Reviewer #2 one-by-one:\nComment 1: One core performance of the device is very narrow Brillouin gain bandwidth\n(3.4MHz). However, there is no clear explanation on this. Why is the phonon lifetime so long\nin this chip? Is it related to the material properties of LN and sapphire?\nResponse: We appreciate the Reviewer for this question, which addresses a critical aspect of\nthe device's performance limits.\nThe reviewer is correct. As indicated by Eq. S.16 in Supplementary Information, without\nconsidering losses (α =α=α=0), we have Δβ ≈ 5.56/L, where Δβ is proportional to the gain\np i s\nbandwidth and L denotes the interaction length. It implies that we can achieve narrower spectral\nbandwidths provided we continually increase the interaction length, if the acoustic loss is\nnegligible. However, we found the bandwidth saturated for a device length longer than 1 mm.\nTherefore, the experimentally observed 3.4 MHz linewidth is not limited by the interaction\nlength but is instead dominated by the lifetime of the acoustic phonons within the waveguide.\nThis finite lifetime ultimately determines the spectral selectivity of the Brillouin interaction. In\nthe main paper we directly addresses the limited spectral resolution resulting from this finite\nphonon lifetime as noted on page 6: \"...we find that the practical resolution of our spectrometer\ndoes not improve with further increases in L, revealing the constraints imposed by the\npropagation loss of the phononic mode in the waveguide\".\nThis physical limitation is quantitatively modeled in Figure 4b of the main manuscript (the\nfigure below). The figure presents simulation results of the device resolution as a function of\ninteraction length L for different phononic quality factors (Q). The red line, which corresponds\nto a Q-factor of approximately 3000, accurately reproduces the experimentally observed\nsaturation of resolution for interaction lengths greater than 1 mm. This Q-factor is entirely\nconsistent with the independent experimental characterization detailed in the Supplementary\nInformation, Section III.B, which estimates the phonon Q-factor to be in the range of 2000–\n4000.\nAs the Reviewer mentioned, the long lifetime of our phonon is fundamentally enabled by our\nchoice of a single-crystal thin-film lithium niobate on a sapphire substrate, a material system\nselected for its ability to support low-loss phononic waveguides. Crucially, this high Q-factor\nis not an assumption but an experimentally verified parameter.\nComment 2: The authors also suggested the acoustic Q factor could be increased by 7-folds,\nbut there is no detail on how to. They just added a reference 38 which is on bulk LN (not\nthin film LN) – I could not find any clue how to improve phonon-Q.\nResponse: We appreciate the reviewer's insightful comment. For an observed phonon Q-factor\nin our work, it should be determined mainly by three factors: (1) the intrinsic material\nabsorption, (2) the defects in the materials, and (3) the surface roughness due to fabrication.\nAccording to Ref. [38], at our operating frequency of 8.5 GHz, the intrinsic acoustic quality\nfactor of the material can reach 20,000. Therefore, the phonon-Q due to the intrinsic material\nabsorption should be even better than 20,000 in this bulk single-crystal LN structure. We agree\nwith the reviewer that original reference [38] discusses acoustic attenuation in bulk LN, not\nthin-film waveguides. The purpose of citing this work is not to propose specific methods for\nimproving acoustic Q-factor, but rather to provide an lower limit for acoustic Q-factor set by\nthe inherent properties of lithium niobate materials itself.\nThe discrepancy between this potential value and the Q-factor of ~3000 measured in our device\nstrongly indicates that our current performance is limited by extrinsic loss mechanisms\nintroduced during fabrication, rather than by the material itself. These extrinsic losses arise\nfrom two primary sources:\n1) The imperfections arising from the wafer production process. The commercial LN-on-\nsapphire films are typically produced via an ion-slicing method, which is known to\nintroduce residual lattice damage that acts as a scattering center for phonons.\n2) The surface scattering from the etched imperfect sidewalls of the waveguide. Because our\nphononic mode is tightly confined, it interacts strongly with these surfaces, meaning any\nfabrication-induced roughness causes significant phononic scattering loss.\nConsidering these factors, performance improvements can be achieved by optimizing\nnanofabrication processes to minimize the surface scattering-related losses. Furthermore, to\nreach the ultimate quality factor constrained by the intrinsic properties of the materials, future\ndevelopments in thin-film material preparation are needed, such as advancements in wafer\nbonding techniques and defect repair technologies. We add a paragraph in Supplement VI part\nto discussing this as following:\n“In addition to expanding the working bandwidth, further improvements lie in enhancing the\nintrinsic resolution of the spectrometer by increasing the phonon quality factor. Current devices\nexhibit a Q-factor of approximately 3000. We believe this value is not constrained by the\nintrinsic acoustic properties of single-crystal lithium niobate but rather by external loss\nmechanisms. The significant difference between the experimentally measured quality factor and\nthe higher Q value derived from bulk material studies indicates substantial room for\nimprovement. These external losses primarily arise from two sources: inherent defects in the\nthin-film wafers and defects introduced during the nanofabrication process.\nTherefore, future performance enhancements can be realized through two approaches:\nsystematically optimizing nanofabrication processes to reduce surface scattering losses, and\nfurther improving the thin-film material preparation techniques, such as enhancing wafer\nbonding processes and developing defect repair or annealing technologies suitable for this\nmaterial system. Successfully addressing these challenges will pave the way for the emergence\nof a new generation of ultra-high-resolution chip-scale Brillouin spectrometers.”\nComment 3: The platform is Zhengfu architecture–LN on Sapphire. The authors did not\nexplain why they use this specific combination. One trivial question would be whether\nconventional thin film LN on SiO2/Si can be used for this application. If it works, the authors\nexplain why they use this architecture. If not, the authors explain why the conventional platform\nis not working.\nResponse: Thank you for your insightful comment. The choice of substrate is a critical design\ndecision for our hybrid photonic-phononic platform, and we are pleased to have this opportunity\nto clarify the reasons behind it. The Zhengfu architecture has three main advantages: (1) tight\noptical and acoustic wave confinement in phononic waveguide, (2) low acoustic and optical\nlosses in single-crystal materials, and (3) the piezoelectricity of the LN material to enable the\nefficient conversion between microwave and acoustic wave. These advantages make this\narchitecture being an idea platform for realizing the hybrid electric-phononic-photonic devices\nfor advanced applications.\nFor the potential candidate of the substrate, i.e., the traditional thin-film lithium niobate\narchitecture based on silicon dioxide (SiO₂)/Si platforms mentioned by the Reviewer, the SiO₂\nlayer has significant acoustic loss. The performance of our spectrometer relies on the efficient\ngeneration and low-loss propagation of high-frequency (~8.5 GHz) acoustic waves (phonons).\nThe SiO₂ buried oxide layer in standard LNOI wafers is an amorphous material exhibiting\nextremely high acoustic attenuation at the GHz frequencies we utilize. Any acoustic leakage\nfrom the LNOI waveguide into the SiO₂ layer will result in a drastically reduced phonon\nlifetime. In contrast, sapphire exhibits inherently low acoustic loss as a single-crystal material.\nAs discussed in above Reponses, a high phonon Q-factors is critical for achieving high spectral\nresolution.\nComment 4: It is clear the device can measure the spectrum of input lights. However, I am\nnot convinced on the accuracy of light intensity measurement. At a single wavelength, the\nreflected optical power has a linear relation (in log-scale) with input power. The linear\ncoefficients at different wavelengths are the function of the frequency-dependent RF-phonon\nconversion efficiency, which is determined by the bandwidth of the IDT (yellow shaded area in\nFig3-a). However, I found that the reflected light powers are not following the conversion\nefficiency curve. In this situation, it would be very difficult to guess the power of input light at\ndifferent wavelengths.\nResponse: We thank the Reviewer for this sharp observation. The reviewer is correct that the\nmeasured reflection peak in Figure 3a (the figure below) does not perfectly align with the\nidealized IDT efficiency envelope. However, this deviation does not represent a measurement\nerror. Actually, we have repeated the measurement many times, and the results are reproducible.\nTherefore, the stable response of the spectrometer at different microwave frequency indicates\nthat standard calibration procedures must be performed on the system.\nThe reasons for the observed bandwidth difference lie in the fact that the final optical reflected\npower results from the combined effects of multiple wavelength-dependent mechanisms, not\nsolely the electro-acoustic conversion response of the IDT. Key influencing factors include:\n1) Wavelength-dependent Acousto-Optic Coupling: The intrinsic coupling strength of the\nBrillouin interaction itself varies with the optical wavelength, directly affecting the\nreflection efficiency.\n2) Photonic Multiplexer Response: The on-chip phononic-photonic multiplexing device,\nwhich acts as a directional coupler to guide light into the interaction region, has a\nwavelength-sensitive transfer efficiency.\n3) Polarization-Dependent Coupling: The system exhibits high sensitivity to input\npolarization state, primarily due to two factors: First, the Brillouin interaction is\nspecifically designed for the fundamental TE mode; Second, the grating couplers used\nfor input/output exhibit both polarization-dependent and wavelength-dependent\nresponse characteristics. Light output from standard input fibers cannot maintain a\nconstant polarization state across the entire spectrum. Despite manual optimization of\ncoupling using a polarization controller, residual fluctuations cause the optical power\ninjected into the target chip's TE mode to vary with wavelength. This directly results\nin fluctuations in the system transfer function. In practical applications, this issue can\nbe addressed by employing a fully polarization-maintaining optical path.\nThe key point is that this complex transfer function, arising from the combination of all these\neffects, is stable and repeatable. The data in Figure 3a is therefore not a flaw, but rather the\nessential dataset used for calibration. As described in the Supplementary Information (Section\nIV), we use this data to construct a calibration matrix M that empirically captures the complete\nsystem response. Accurate intensity measurements of an unknown source are then achieved by\nmeasuring its raw output and mathematically applying this calibration matrix to retrieve the\ntrue input spectrum. The high agreement with commercial spectrometers shown in Figure 3c\n(the figure below) strongly demonstrates that this standard calibration procedure enables the\ndevice to perform precise intensity measurements across its operational bandwidth.\nTo clarify this point, we add a sentence in the main text in page 5:\n“Figure3 demonstrates the broadband performance of our spectrometer……. It is worth noting\nthat the observed fluctuations on this envelope reflect the complete, wavelength-dependent\nsystem transfer function, which includes not only the IDT response but also factors such as the\nintrinsic acousto-optic coupling strength and on-chip polarization effects. This entire\nrepeatable response is captured in the calibration matrix used for spectral reconstruction.”\nResponse to Reviewer #3\nGeneral comment: The manuscript titled “Compact and high-resolution spectrometer via\nBrillouin integrated circuits” demonstrates a waveguide-based device integrated with\ninterdigital transducer to excite acoustic waves which then scatter the input optical signal. The\nstudy proposes its use as a compact spectrometer.\nResponse: We sincerely thank Reviewer #3 for the careful reading of our manuscript and for\nraising important questions regarding the fundamental operating principle and novelty of our\nwork. We appreciate the opportunity to clarify these critical distinctions.\nWe note that the other expert reviewers have recognized the fundamental novelty and\nsignificance of our approach. Reviewer #1 specifically stated: “As far as I can tell, this is the\nfirst demonstration of IDT-induced on-chip Brillouin scattering for spectroscopy” and\nconcluded that “this technology could have significant commercial impact for compact\nprecision optical spectroscopy.” and that the work “could warrant publication in Nature\nCommunications.” Similarly, Reviewer #2 acknowledged that “the principle of spectrometer is\nrather simple, coming from the linear relationship between Brillouin shifts and pump\nwavelengths” and concluded the paper “could be quite interesting to relevant community.”\nThese assessments reflect recognition of our key innovation: we demonstrate the first chip-\nscale spectrometer based on electronically-controlled acoustic pumping of Brillouin scattering,\nfundamentally distinct from all prior Brillouin-based optical systems.\nIn the following, we address the comments raised by Reviewer #3 one-by-one:\nComment 1: I do not see it any different from a pump-probe Brillouin scattering experiment\nin an integrated waveguide, where tuning the pump wavelength results in tuning of the Stokes\nfrequency at which maximum gain would occur.\nResponse: We sincerely appreciate the Reviewer’s comments. We believe there is a\nfundamental misunderstanding of the working principle of our device, which is distinct from a\nconventional pump-probe stimulated Brillouin scattering (SBS) experiment. Just as the other\ntwo expert reviewers pointed out this crucial distinction in their comments:\n Reviewer #1 accurately described our method, noting, \"the GHz acoustic wave is\ngenerated by an on-chip IDT device directed into the waveguide\" and concluded that this\nis \"the first demonstration of IDT-induced on-chip Brillouin scattering for spectroscopy.\"\n Reviewer #2 correctly highlighted the use of an \"external IDT on LiNbO3 at around\n8.5GHz.\"\nThe core innovation of our work is that our device operates as an acoustically-pumped\nspectrometer. Unlike conventional pump-probe Brillouin experiments that require a strong,\nknown optical pump laser to mediate the interaction, our method requires no optical pump\nwhatsoever. The only optical input is the unknown signal to be measured. The role of the\n\"pump\" is fulfilled by a tunable, electronically-generated acoustic wave, which stimulates the\nreflection of the input signal. To our knowledge, this is the first demonstration of a spectrometer\nbased on this purely acoustically-driven principle.\nFurthermore, our device specifically leverages backward Brillouin scattering, a configuration\nwhich maximizes the photoacoustic interaction and allows the spectrometer's resolution to\napproach the fundamental limit for a given device length, as explained in the manuscript. This\nunique combination of acoustic drive mechanisms with high-resolution backscattering\ngeometry is the core feature of our work.\nComment 2: Further, the input power of the optical signal has to be above a few hundreds\nmicrowatts for the device to generate a detectable backscattered signal with good signal-to-\nnoise ratio, which places a minimum threshold on the power of the optical signal to be detected.\nResponse: We thank the reviewer for raising this issue. We would like to clarify that the device\noperates at lower power levels than suggested. The inset of Figure 3a in the main manuscript\n(as shown in the figure below) explicitly demonstrates that the linear relationship between input\nand reflected power holds down to the lowest measured input power of 100 µW. The reflect\nratio shown in Figure 3a indicates that over 50% input signal can be reflected and then\nmeasured by our detector using direct photodetection or heterodyne detection scheme. So the\nlimitation of our device signal-to-noise ratio (SNR) arises from imperfections in the detection\nmodule and the optical coupling interfaces, rather than from the Brillouin process itself. The\nSNR for measuring even weaker signals can be further enhanced by increasing the scanning\ntime or the number of averages.\nComment 3: Further, it will be better to compare the input spectrum using the both commercial\nspectrometer and the proposed device using the same scale (log or linear).\nResponse: We thank the Reviewer for this nice suggestion. We agree that presenting both\nspectra on a common scale would provide a more direct and quantitative comparison. In the\nrevised version of the manuscript, we have updated Figure 3c, which plot the spectrum data\nfrom both our device and the commercial spectrometer on a normalized linear scale to better\nillustrate the high fidelity of our instrument. The updated Figure 3c is shown as below:\nComment 4: The resolution of 0.56 nm achieved in this demonstration is much lower than what\nhas been obtained using the Brillouin dynamic gratings in optical fiber (Optics Letters Vol. 39,\nIssue 10, pp. 2967-2970 (2014)) or photonic chip (Optics Letters Vol. 38, Issue 3, pp. 305-307\n(2013)).\nResponse: Thank you for your comment. We would like to respectfully clarify that a direct\ncomparison of resolution with the cited works is not appropriate, because our device operates\non a fundamentally different principle and serves a distinct function as a spectrometer.\nRegarding the working principle, the devices mentioned by the Reviewer rely on optically\npumped stimulated Brillouin scattering (SBS). Such systems require (1) high pump laser power\nand (2) a calibrated laser for signal detection. For a device to function as a universal\nspectrometer, it is impractical and fundamentally limiting to rely on powerful external\ncalibrated lasers to measure unknown input signals.\nOur acoustically pumped device, in complete contrast, is a self-contained spectrometer that uses\na purely electronic (RF) signal to generate the acoustic grating. This allows it to directly analyze\nany compatible input optical signal without needing another optical source, which is the core\ncharacteristic of a true spectrometer. Given this fundamental difference in operational principle\nand application scope, we believe that a comparison based on a single performance metric such\nas resolution is not meaningful.\nFurthermore, spectral resolution is intrinsically related to the interaction length (i.e., footprint)\nof the device, as dictated by fundamental physics. The primary contribution of our work lies in\nachieving unprecedentedly high resolution within a compact device. As shown in Figure 4a (as\nshown below), through a comprehensive comparison with other state-of-the-art integrated\nspectrometers, our device sets a new performance benchmark within a millimeter-scale\nfootprint. We believe this is the most critical metric for evaluating the significance and\nbreakthrough nature of our research in the field of integrated photonics."}, {"round": 2, "reviewer_comments": {"reviewer_1": "I am satisfied with the authors' responses to my comments and would be happy for the paper to proceed to publication.", "reviewer_2": "The authors successfully addressed all the points I raised; therefore the manuscript is publishable.", "reviewer_3": "I see the point made by authors about on-chip excitation of acoustic waves using IDT and the chart comparing the resolution\nperformance of the device is useful. The constraint on the signal power makes it useful only for applications where the signal\npower is more than 100 uW (-10 dBm), as also mentioned by the authors in the response letter.\nMost of the commercial spectrometer, be it grating based or otherwise, can operate at signal powers of ~ nW or less. The\ncurrent device requires 5 orders larger power for detection and even at that power the reflected power is very less as can be\nseen from Fig. 3(a). How does the required signal power in this work compare with other on-chip spectrometer\ndemonstrations?\nIn the Introduction, authors have mentioned “capability of PIC in spectroscopic analysis for astronomy measurements [10,\n11], medical diagnostics [12], environmental monitoring [13], and chemical analysis [14, 15], high-performance optical\nspectrometers are indispensable [16–18].”\nWhat is the magnitude of the signal power in astronomy measurements, environmental monitoring and other applications\nmentioned above? My understanding is that light scattered from tissues or coming from stars or scattered from atmospheric\ngases is much less than that required for detection using the device.\nCould the authors explain, how the device can be used in these applications, where light have to be first collected from free\nspace and then coupled into waveguide for scattering from the acoustic waves?"}, "author_response": "ResponsetoReferee#1\nGeneral comment: I am satisfied with the authors' responses to my comments and would be\nhappyforthepapertoproceedtopublication.\nResponse: We thank Referee #1 for the recognition of our paper. We are also very grateful\nfor the insightful comments and constructive suggestions, which have helped us to\nsignificantlyimprovethispaper.\nResponsetoReferee#2:\nGeneral comment: The authors successfully addressed all the points I raised; therefore the\nmanuscriptispublishable.\nResponse: We thank Referee #2 for the recognition of our paper. We also appreciate the\nReferee for raising valuable comments for the purpose of clarifying and improving our\nmanuscript.\nResponsetoReferee#3:\nComment 1: I see the point made by authors about on-chip excitation of acoustic waves\nusing IDT and the chart comparing the resolution performance of the device is useful. The\nconstraintonthesignalpowermakesitusefulonlyforapplicationswherethesignalpoweris\nmorethan100uW(-10dBm),as alsomentionedby theauthorsintheresponse letter.Mostof\nthe commercial spectrometer, be it grating based or otherwise, can operate at signal powers\nof ~ nW or less. The current device requires 5 orders larger power for detection and even at\nthat power the reflected power is very less as can be\nseen from Fig. 3(a). How does the requiredsignalpowerinthisworkcomparewithotheron-chipspectrometerdemonstrations?\nResponse:WethankReferee#3fortherecognitionofourpaper.Herewewouldliketo\nrestatethesignal-to-noiseratio(SNR)ofourdevice.ThereflectratioshowninFigure3a\nrepresentsthatover50%inputsignalcanbereflectedandthenmeasuredbyourdetector\nusingdirectphotodetectionorheterodynedetectionscheme.Sothelimitationofourdevice\nSNRarisesfromimperfectionsinthedetectionmoduleandtheopticalcouplinginterfaces,\nratherthanfromtheBrillouinprocessitself.AndtheSNRformeasuringevenweakersignals\ncanbefurtherenhancedbyincreasingthescanningtimeorthenumberofaverages.\nComment 2: In the Introduction, authors have mentioned “capability of PIC in\nspectroscopic analysis for astronomy measurements [10, 11], medical diagnostics [12],\nenvironmental monitoring [13], and chemical analysis [14, 15], high-performance optical\nspectrometers are indispensable [16–18].”What is the magnitude of the signal power in\nastronomymeasurements,environmentalmonitoringandotherapplicationsmentionedabove?\nMy understanding is that light scattered from tissues or coming from stars or scattered from\natmospheric gases is much less than that required for detection using the device.Could the\nauthorsexplain,howthedevicecanbe usedintheseapplications,wherelighthavetobefirst\ncollected from free space and then coupled into waveguide for scattering from the acoustic\nwaves?\nResponse:WethanktheRefereeforraisingthisissue.TheRefereeiscorrectthatthe\napplicationsmentionedinourintroduction,suchasastronomicalmeasurements,medical\ndiagnosticsviaRamanspectroscopy,andcertaintypesofenvironmentalmonitoringor\nchemicalanalysis,oftendealwithextremelylowsignalpowers(frompicowattstofemtowatts,\norevensingle-photonlevels).\nOurprimarypurposeincitingreferences[10-18]wastoestablishthebroadscientificand\nindustrialmotivationfordevelopinghigh-performance,miniaturizedopticalspectrometers,\nforwhichacompactfootprintandhighresolutionareindispensable.Ourcurrentwork\nfocusesondemonstratingtheprincipleandarchitectureofthisnovel,compactspectrometer,\nwhichleveragesacoustically-stimulatedBrillouinscattering.ThereflectratioshowninFigure\n3arepresentsthatover50%inputsignalcanbereflectedandthenmeasuredbyourdetector\nusingdirectphotodetectionorheterodynedetectionscheme.Sothelimitationofourdevice\nSNRarisesfromimperfectionsinthedetectionmoduleandtheopticalcouplinginterfaces,\nratherthanfromtheBrillouinprocessitself.\nWefullyagreewiththeRefereethatapplyingthisdevicetolow-lightdomainslikeastronomy\nisafuturegoalthatrequiressignificantsystem-levelintegration,mostnotablytheintegration\noflow-noisecomponentsandhigh-sensitivitydetectors,suchasSNSPDs,whichourplatform\niscompatiblewith."}]

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