Generating and Controlling 1064 nm Microcombs with Photonic Bandgap Engineering

Introduction

Microresonator frequency combs, known as microcombs, have emerged as a transformative technology enabling numerous applications across fields like data communications, spectroscopy, and precision frequency synthesis. These chip-integrated optical frequency combs offer compact form factors, wide parametric control, and the ability to be mass-manufactured.

While microcombs in the conventional 1550 nm telecom band have been extensively studied, there is growing interest in exploring microcombs at other wavelength bands. One promising band is around 1064 nm, corresponding to technologically mature components like ytterbium fiber lasers and semiconductor lasers. Accessing this band could enable new applications in areas like bioimaging, spectroscopy of key atomic species, and photonic integration with quantum technologies.

In this tutorial, we will discuss the recent demonstration of 1064 nm microcombs using photonic bandgap engineering in high-Q tantala (Ta2O5) microresonators. We will cover the working principles, experimental realization, theoretical modeling, and future prospects of this approach [1].

Microcombs from Photonic Bandgap Phase-Matching

The formation of microcombs relies on parametric frequency conversion driven by the optical Kerr nonlinearity in the microresonator. Typically, this process requires phase-matching between the pump laser and the resonator's dispersion profile to efficiently generate new frequency components. For anomalous group velocity dispersion (GVD) resonators, the phase-matching is relatively straightforward as the resonances increase in frequency spacing with wavelength. However, achieving phase-matching in the normal GVD regime is challenging since the resonances become more densely spaced at shorter wavelengths.

A powerful approach to enable phase-matching in normal GVD resonators uses photonic crystal structures to induce a photonic bandgap in the resonator mode family. As illustrated in Figure 1a, this involves patterning the microresonator waveguide with a sinusoidal modulation that has a period matched to the specific microresonator mode excited by the pump laser. The bandgap splits this mode into two distinct resonances separated in frequency. By pumping the lower frequency component, the system can phase-match to shorter wavelength resonances beyond the pump, thereby enabling broadband comb generation even in the normal dispersion regime.

Figure 1 describes the creation of 1064 nm dark soliton microcombs using a Photonic Crystal Ring (PhCR). It illustrates (a) the PhCR setup, generating backward-propagating solitons with an inset showing its modes and the pump mode influenced by the photonic crystal. (b) The microcomb's spectrum covers 100 nm with a 200 GHz repetition rate, highlighting the normalization for on-chip power and the phase-matching process through the Kerr effect. (c) The experimental procedure uses a 1064 nm laser, amplified by a ytterbium-doped fiber, to pump the PhCR; the resulting comb is analyzed with an optical spectrum analyzer, with power and transmission measured via a photodetector on an oscilloscope. (d) The conditions for soliton existence are modeled, showing that the necessary photonic bandgap widens with increasing pump field amplitudes, with soliton spectra examples provided for a specific pump amplitude line.

Experimental Realization

The researchers fabricated air-clad tantala microresonators with integrated photonic crystal structures designed for 1064 nm operation. Tantala (Ta2O5) is an attractive platform due to its high linear and nonlinear optical coefficients across the visible and near-IR spectrum.

Figure 1b shows the spectrum of a photonic bandgap microcomb in one of these resonators. Despite operating in the normal dispersion regime, a broadband comb spanning over 100 nm is generated with a repetition rate of 200 GHz. This comb exists in the form of a "dark soliton" pulse circulating in the resonator, in contrast to the bright soliton pulses typically formed in anomalous dispersion resonators.

The experimental setup (Figure 1c) uses a continuous-wave 1064 nm laser amplified by a ytterbium-doped fiber amplifier to pump the on-chip microresonator via a circulator. The backward-propagating comb is then extracted and analyzed using an optical spectrum analyzer. Photodetection allows monitoring the temporal dynamics of comb formation.

Modeling and Design Considerations

To better understand the operating principles and design tradeoffs, the researchers employed numerical modeling based on the coupled Lugiato-Lefever equations. These model the spatio-temporal dynamics of the intracavity fields, including effects like dispersion, nonlinearity, and the photonic bandgap perturbation.

One key study explores how the existence range for stable comb formation depends on the bandgap strength and pump power, as shown in Figure 1d. Larger bandgaps and higher pump powers require increased bandgap splitting to achieve phase-matching, up to a limit where no stable combs can form.

The modeling also reveals how the comb bandwidth can be controlled by the resonator's dispersion profile (via the waveguide geometry) and the induced bandgap strength. As depicted in Figures 2a and 2b, larger bandgaps enable phase-matching to more resonances and thus wider comb bandwidths. Similarly, reducing the GVD by using narrower waveguides relaxes the phase-matching condition, allowing the same bandgap to generate more spectrally broad combs.

Figure 2 explores control over the PhCR microcomb spectrum. (a) It shows the comb power spectrum modeled against changes in the photonic crystal's bandgap, with a fixed pump field amplitude (F = 3) and dispersion (D2 = −3 MHz). Insets demonstrate that while the integrated dispersion remains stable, the mode affected by the photonic crystal shifts upward. (b) This figure also presents the comb power spectrum's dependence on dispersion D2, for a constant pump amplitude and a photonic bandgap set to ten half-linewidths. Insets highlight how integrated dispersion varies with D2 magnitude, without altering the bandgap, alongside simulated comb profiles for specific bandgap and dispersion values.

Experimental Tunability

Returning to the experiments, the researchers demonstrated bandwidth control by fabricating resonators with different waveguide widths to access different dispersion regimes. Figure 3a shows three example comb spectra generated from resonators with distinct GVDs and photonic bandgaps.

Interestingly, the researchers also explored how the combs could be dynamically shaped in their spectral envelope by controlling an effect known as the "breathing instability." This manifests as slow oscillations of the intracavity circulating pulse, causing a time-varying distortion of the output comb spectrum, as seen in the example of Figure 3b. However, by properly adjusting the pump laser detuning, this breathing instability could be eliminated, yielding a clean comb spectrum suitable for applications needing low noise operation.

Figure 3 focuses on adjusting the bandwidth of PhCR microcombs at 1064 nm. It includes (a) experimental spectra of combs with varying combinations of dispersion (D2) and photonic bandgap, which influence the combs' bandwidth, with the data normalized for on-chip power visibility, and (b) an analysis of how altering the breather frequency modifies the comb spectrum.

Outlook and Applications

This work highlights the power of dispersion engineering with photonic bandgaps to generate microcombs across non-conventional wavelength bands like 1064 nm. Potential applications include:

• Bioimaging and medical diagnostics, leveraging the improved tissue penetration of 1064 nm light compared to shorter visible wavelengths.

• Spectroscopy of atomic species with transitions in the 1064 nm region, such as rubidium, cesium, and strontium. This could enable chip-scale optical atomic clocks and quantum sensors.

• Precision frequency synthesis and optical timing, by leveraging 1064 nm as a channel for robust supercontinium generation and f-2f self-referencing.

Looking ahead, further increasing the efficiency and reducing the pump power requirements will be critical for realizing practical applications. Approaches like improved waveguide coupling designs, employing pump recycling techniques, and novel phase-matching schemes could pave the way.

Moreover, adapting this photonic bandgap approach to other wavelength bands in the near-IR and visible ranges could catalyze a wide array of integrated photonic technologies interfaced with atomic, molecular, and semiconductor systems across the optical spectrum.

In summary, the demonstration of 1064 nm photonic bandgap microcombs exemplifies the remarkable versatility and design freedom offered by microresonator frequency combs. With further research extending such dispersion control to other wavelength ranges, we can expect microcombs to keep revolutionizing many areas of science and technology in the years to come.

Reference

[1] Grisha Spektor, Jizhao Zang, Atasi Dan, Travis C. Briles, Grant M. Brodnik, Haixin Liu, Jennifer A. Black, David R. Carlson, Scott B. Papp; Photonic bandgap microcombs at 1064 nm. APL Photonics 1 February 2024; 9 (2): 021303. https://doi.org/10.1063/5.0191602