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Integrated electro-optic isolator on thin-film lithium niobate

Nature Photonics volume 17, pages 666–671 (2023)Cite this article

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Abstract

Optical isolators are indispensable components of almost any optical system and are used to protect a laser from unwanted reflections for phase-stable coherent operation. The emergence of chip-scale optical systems, powered by semiconductor lasers that are integrated on the same chip, has generated a demand for a fully integrated optical isolator. Conventional approaches, which rely on the use of magneto-optic materials to break Lorentz reciprocity, present substantial challenges in terms of material integration. Although alternative magnetic-free approaches have been explored, an integrated isolator with a low insertion loss, high isolation ratio, broad bandwidth and low power consumption on a monolithic material platform is yet to be achieved. Here we realize a non-reciprocal travelling-wave-based electro-optic isolator on thin-film lithium niobate. The isolator enables a maximum optical isolation of 48.0 dB with an on-chip insertion loss of 0.5 dB and uses a single-frequency microwave drive power of 21 dBm. The isolation ratio remains larger than 37 dB across a tunable optical wavelength range from 1,510 to 1,630 nm. We realize a hybrid distributed feedback laser–lithium niobate isolator module that successfully protects the single-mode operation and linewidth of the laser from reflection. Our result represents an important step towards a practical high-performance optical isolator on chip.

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Fig. 1: Concept of an integrated EO isolator.
Fig. 2: Characterization of isolation and optical bandwidth of the EO isolator.
Fig. 3: Integration of a tunable add–drop ring filter on one TF LN chip.
Fig. 4: Characterization of the DFB laser and isolator module.

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Data availability

The datasets generated and analysed in the current study are available from the corresponding authors on reasonable request.

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Acknowledgements

This work is supported by the Defense Advanced Research Projects Agency HR0011-20-C-0137 (M.Y., R.C., C.R., L.H., K.L., L.S., A.S.-A., H.R.G., L.J., M.Z. and M.L.), ONR N00014-18-C-1043 (R.C. and M.Y.) and N00014-22-C-1041 (R.C. and M.Y.), AFOSR FA9550-19-1-0376 (A.S.-A.) and Raytheon Technology A40210 (L.S.). E.P. acknowledges support by the Draper graduate student fellowship program. The device fabrication was performed at the Harvard University Center for Nanoscale Systems. The views, opinions and/or findings expressed are those of the authors and should not be interpreted as representing the official views or policies of the Department of Defense or the US government.

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Author notes

  1. These authors contributed equally: Mengjie Yu, Rebecca Cheng.

Authors and Affiliations

  1. John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA

    Mengjie Yu, Rebecca Cheng, Eric Puma, Linbo Shao, Amirhassan Shams-Ansari & Marko Lončar

  2. Ming Hsieh Department of Electrical and Computer Engineering, University of Southern California, Los Angeles, CA, USA

    Mengjie Yu & Xinyi Ren

  3. HyperLight, Cambridge, MA, USA

    Christian Reimer, Lingyan He, Kevin Luke & Mian Zhang

  4. Draper Scholar, The Charles Stark Draper Laboratory, Cambridge, MA, USA

    Eric Puma

  5. Bradley Department of Electrical and Computer Engineering, Virginia Tech, Blacksburg, VA, USA

    Linbo Shao

  6. Freedom Photonics, Goleta, CA, USA

    Hannah R. Grant & Leif Johansson

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Contributions

M.L. conceived the idea. M.Y. designed the chip with the help of R.C., C.R., L.H. and M.Z. C.R., K.L. and L.H. fabricated the devices. M.Y. and R.C. carried out the measurement and analysed the data with help from E.P., L.S., A.S. and X.R. M.Y. performed the numerical simulations. H.G. and L.J. provided the DFB laser. M.Y. and R.C. wrote the manuscript with contribution from all authors. M.L. supervised the project.

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Correspondence to Mengjie Yu or Marko Lončar.

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Competing interests

C.R., K.L., L.H., M.Z. and M.L. are involved in developing LN technologies at HyperLight Corporation. The remaining authors declare no competing interests.

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Nature Photonics thanks Yeshaiahu Fainman, Juejun Hu and Changzheng Sun for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Full measurement setup for the LN electro-optic isolator.

Forward and backward spectra of the isolator are monitored using a tunable laser and 2×2 switch (forward: 1 → 3, backward: 1 → 4) to couple light onto the chip. Polarization controllers are used in both directions to couple light into the transverse-electric (TE) waveguide mode. Spectra are measured on an OSA (optical spectrum analyzer). For III-V laser operation, the DFB laser is edge-coupled directly to the isolator chip. The output is then split between a retroreflector and linewidth and relative intensity noise (RIN) measurement to monitor the behavior of the DFB laser with and without isolation.

Extended Data Fig. 2 Stability measurement of the isolation ratio.

a, The isolation ratio as a function of microwave power. The simulation is performed using the fundamental Bessel function \({J}_{0}(\beta )\), where the microwave power is proportional to \({\beta }^{2}\). The highest simulated isolation ratio is limited by the finite sampling points. The experimental data shows excellent agreement with the simulation when the microwave power is detuned more than 0.05 dBm ( ~ 1.1%) from the optimal power. b, The measured isolation ratio over 14 hours at a time interval of 30 seconds. At the beginning, the isolation of 46 dB is achieved at an optical wavelength of 1555 nm and similar microwave setting to that in Fig. 2a & b in the main text (inset: the optical spectrum). To assess the effects of environmental conditions (temperature, air fluctuation, acoustic noise etc), the isolation ratio is monitored as a function of time while keeping all the operation conditions the same. The isolation gradually decreases to 41 dB after 2 hours and stabilizes to a mean value of 41.3 dB with a standard deviation of 0.56 dB over a 12-hour period. The primary reason for the drop in isolation is the polarization variation over time, which is confirmed by the fact that the isolation ratio can be adjusted back to 46 dB after manually adjusting the polarization controller in the setup.

Extended Data Fig. 3 Coupling between DFB laser and LN chip.

a, Quantum well waveguide structure and (inset) simulated output mode of the III-V DFB laser: W = 5 um, HP = 2.5 um, h­P = 500 nm, HQW = 450 nm, HN = 4 um; b, Structure of the LN mode converter at the coupling facet: h = 280 nm, w = 250 nm; c, Simulation of the coupling efficiency between the DFB laser mode and LN facet. An anti-reflective coating is added to the laser structure, as in experiment, to minimize reflection between the laser and the air gap. The minimum coupling gap achievable in our experiment is approximately 2 um, corresponding to about 2-dB coupling loss (indicated by the star). This is loss is further verified experimentally on other LN chips.

Extended Data Fig. 4 Optical spectral measurement of the DFB laser - LN isolator module.

The optical spectrum of the DFB laser module when the isolator is turned on and off.

Extended Data Fig. 5 Relative intensity noise (RIN) measurement with the LN-chip-based isolator.

a, The RIN measurement under four different conditions: 1) isolator on, reflection off; 2) isolator on, reflection on; 3) isolator off, reflection off; 4) isolator off, reflection on. The reflected power back into the DFB chip is estimated to be 0.4% (24 dB) of the laser emission power Plaser. The spectral range of the measurement is from DC to 8 MHz. The detector bandwidth is 200 MHz. We observe similar RIN level between the integrated isolator turned on and off while the reflection is turned off. When both reflection and isolator turned on, we observe an increase in RIN. The cause behind this phenomenon is still under investigation since the laser is followed by an extra cavity which is formed between the reflector and chip output facet. When the reflection is turned on and the isolator is turned off, the DFB laser is not protected, and we observe beatnotes at harmonics of the external cavity’s FSR (purple curves in a and b). b, The RIN measurement for the case of isolator off and reflection on. The spectral range of the measurement is from DC to 22 MHz so we could see the higher harmonics of the beatnotes. In this measurement, the FSR of the external cavity is about 7.2 MHz. iso: isolator; ref: reflection.

Extended Data Fig. 6 The RIN and linewidth measurement using a fiber-based isolator as a benchmark.

The isolation ratio is 50 dB. The reflected power is also set at 0.4% of the laser power. a, The RIN measurement over 8 MHz range under four different conditions: 1) isolator on, reflection off; 2) isolator on, reflection on; 3) isolator off, reflection off; 4) isolator off, reflection on. The isolator successfully protects the RIN of the laser. Only in the case of reflection on and isolator off, we observe the beatnote which corresponds to the external cavity FSR (similar to what is observed in the Extended Data Fig. 5). b, The linewidth measurement of the laser. We observe a similar phenomenon as compared to the LN-chip-based isolator case plotted in Fig. 4c. The isolator can protect the single mode operation and the linewidth of the laser. Without the isolator’s protection, the reflection would cause multimode oscillation with spectral spacing equal to the FSR of the external cavity formed between the DFB laser and the fiber reflector. In this measurement, the cavity FSR is 7.2 MHz. iso: isolator; ref: reflection.

Extended Data Table 1 Performance comparison with other on-chip isolator approaches
Full size table

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Supplementary Information

Supplementary Figs. 1–5 and Discussion.

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Yu, M., Cheng, R., Reimer, C. et al. Integrated electro-optic isolator on thin-film lithium niobate. Nat. Photon. 17, 666–671 (2023). https://doi.org/10.1038/s41566-023-01227-8

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