Electro-optic Comb Generation Research Articles

Ultraviolet astronomical spectrograph calibration with laser frequency combs from nanophotonic lithium niobate waveguides

Astronomical precision spectroscopy underpins searches for life beyond Earth, direct observation of the expanding Universe and constraining the potential variability of physical constants on cosmological scales. Laser frequency combs can provide the required accurate and precise calibration to the astronomical spectrographs. For cosmological studies, extending the calibration with such astrocombs to the ultraviolet spectral range is desirable, however, strong material dispersion and large spectral separation from the established infrared laser oscillators have made this challenging. Here, we demonstrate astronomical spectrograph calibration with an astrocomb in the ultraviolet spectral range below 400 nm. This is accomplished via chip-integrated highly nonlinear photonics in periodically-poled, nano-fabricated lithium niobate waveguides in conjunction with a robust infrared electro-optic comb generator, as well as a chip-integrated microresonator comb. These results demonstrate a viable route towards astronomical precision spectroscopy in the ultraviolet and could contribute to unlock the full potential of next-generation ground-based and future space-based instruments.

Single-drive electro-optic frequency comb source on a photonic-wire-bonded thin-film lithium niobate platform.

Stable pulse and flat-top frequency comb generation are an indispensable component of many photonic applications, from ranging to communications. Lithium niobate on insulator is an excellent electro-optic (EO) platform, exhibiting high modulation efficiency and low optical loss, making it a fitting candidate for pulse generation through electro-optic modulation of continuous-wave (CW) light, a commonly utilized method for generating ultrashort pulses. Here, we demonstrate an on-chip electro-optic comb generation module on thin-film lithium niobate (TFLN) consisting of a Mach-Zehnder interferometer (MZI) amplitude modulator (AM) and a cascaded phase modulator (PM) system driven by a single-electrode drive. We show that when operated in the correct regime, the lithium niobate chips can generate frequency combs with excellent spectral power flatness. In addition, we optically package one of the pulse generator chips via photonic wire bonding. The pulses generated by the photonic-wire-bonded device are compressed to 840 fs pulse duration using an optical fiber and show extremely stable operation.

Dual Electro-Optic Comb Generator by Heterodyning Two Combs for Extending the Optical Bandwidth

A novel dual-electro-optic (DEO) comb generator is proposed by heterodyning the optical frequency combs (OFCs) to achieve a high bandwidth of ~2.74 THz with an efficient and cost-effective structure. This can attract the research community’s interest, particularly for the inherent support in optical communications due to its simplicity and high throughput. The OFC is generated by cascading a continuous wave (CW) laser and Mach Zehnder modulator (MZM) coupled to a second laser source that produces a broad spectrum of frequencies by the frequency heterodyning after passing it through a second MZM. This is the highest number of comb lines produced so far in cascaded configurations of the electro-optic modulators. The frequency difference between the two lasers is carefully selected, equal to 1.38 THz, which combinedly produces around 137 well-flattened OFC lines with 20 GHz frequency spacing and more than 40 dBm tone to noise ratio and an amplitude difference of 0 to 6 dB. The bandwidth of the generated flattened comb source is approximately 2.74 THz, which covers over half of the optical C-band.

Time-expanded φOTDR using low-frequency electronics.

Time expanded phase-sensitive optical time-domain reflectometry (TE-φOTDR) is a recently reported technique for distributed optical fiber sensing based on the interference of two mutually coherent optical frequency combs. This approach enables distributed acoustic sensing with centimeter resolution while keeping the detection bandwidth in the megahertz range. In this paper, we demonstrate that TE-φOTDR can be realized with low-frequency electronics for both signal generation and detection. This achievement is possible thanks to the use of a couple of electro-optic comb generators driven by commercially available step recovery diodes. These components are fed by radio frequencies that are orders of magnitude lower than those involved in the signals so far originated by ultrafast waveform generation. The result is a simple, compact, low-cost and potentially field-deployable sensor that works without resorting to any decoding algorithm. Besides, high-resolution distributed sensing is carried out with no need of coding strategies or enhanced backscatter fibers. To check the capabilities of our system, we perform distributed strain sensing over a range of 20 m. The spatial resolution is 3 cm and the acoustic sampling rate can be increased up to 200 Hz. This performance reveals the prospective of the proposed approach for field applications, including structural health monitoring.

Common-Path Dual-Comb Spectroscopy Using a Single Electro-Optic Modulator

Dual frequency comb (DFC) spectroscopy using electro-optic comb generators stands out for its flexibility, easy implementation, and low cost. Typically, two combs with different line spacing are generated from a common laser using independent electro-optic comb generators. This approach minimizes the impact of laser phase noise; however, the distinct paths followed by the two combs ultimately limit the attainable signal-to-noise ratio and long-term stability of the system. In this work, a common-path DFC is generated using a single modulator driven by an arbitrary waveform generator, thus enabling a remarkable increase of the system stability (up to 0.8 s of integration time) while maintaining high flexibility. The proposed technique is experimentally validated by implementing a dual frequency comb with 3000 lines, covering an optical bandwidth of 4.5 GHz, and demonstrating an optical-to-radiofrequency compression factor of 7500. Our system is able to measure extremely narrowband optical features (in the MHz range) with an accuracy only limited by the master laser stability.

Analysis of a highly efficient phase-locking stabilization method for electro-optic comb generation.

We theoretically show that a slightly modified Pound-Drever-Hall (PDH) stabilization scheme can lead to the optimum time-domain characteristics for electro-optic comb generators (EOCG). The ideal locking point is located by analyzing the EOCG output pulse width. By summing up the electrical field reflected by the EOCG front mirror, a model of the phase-locking error signal is derived with the Jacobi-Anger identical transformation. The simulation and experiment show that the zero-locking point of the error signal of the modified scheme coincides well with the ideal locking point in contrast with the direct application of the PDH scheme. Finally, a power efficiency of up to 2.9% is achieved with this EOCG stabilization scheme. A relative instability of better than 2.6×10-8 is demonstrated by a dual comb interferometer with fixed paths. The Allan deviations of the comb mode frequencies are smaller than 2.8×10-9 and 1.1×10-10 for average times of 1 and 100 s, respectively.

Low noise electro-optic comb generation by fully stabilizing to a mode-locked fiber comb.

A fully stabilized EO comb is demonstrated by phase locking the two degrees of freedom of an EO comb to a low noise mode-locked fiber comb. Division/magnification of residual phase noise of locked beats is observed by measuring an out-of-loop beat. By phase locking the 200 th harmonics of the EO comb and a driving cw frequency to a fiber comb, a record low phase noise EO comb across +/- 200 harmonics (from 1544.8 nm to 1577.3 nm) is demonstrated.

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