S. Koptyaev

Soliton dual frequency combs in crystalline microresonators

We present a novel compact dual-comb source based on a monolithic optical crystalline MgF2 multi-resonator stack. The coherent soliton combs generated in the two microresonators of the stack with the repetition rate of 12.1 GHz and difference of 1.62 MHz provided after heterodyning a 300 MHz wide radio frequency comb. An analogous system can be used for dual-comb spectroscopy, coherent LIDAR applications, and massively parallel optical communications.

Method for optical inspection of nanoscale objects based upon analysis of their defocused images and features of its practical implementation.

A microscopic method to inspect isolated sub 100 nm scale structures made of silicon is presented. This method is based upon an analysis of light intensity distributions at defocused images obtained along the optical axis normal to the sample plane. Experimental measurements of calibrated lines (height 50 nm, length 100 μm, and widths of 40-150 nm in 10 nm steps) on top of a monocrystalline silicon substrate are presented. Library of defocused images of calibrated lines is obtained experimentally and numerically with accordance to experimental setup parameters and measurements conditions. Processing of the measured defocused images and comparison with simulated ones from library allow one to distinguish between objects with a 10 nm change in width. It is shown that influence of optical system aberrations must be taken into account in order to achieve coincidence between simulation and measured results and increase accuracy of line width inspection accuracy. The limits of accuracy for object width measurements using this optical method are discussed.

Motion-free all optical inspection system for nanoscale topology control.

We present a novel all optical method for nanoscale pattern inspection. This method uses the chromatic aberration in an imaging optical system and a tunable light source. Such an approach allows stable and precise inspection of nanoscale objects based on an analysis of their defocused diffraction patterns without any external mechanical influence on the sample or optical system. We demonstrate the efficiency of a low cost light source tunable in the range of a light emitting diode bandwidth of ~30 nm (FWHM) for providing the required defocusing. The proposed method is tested using calibrated lines (height 50 nm, length 100 μm, critical dimension (СD) value range 40-150 nm with 10 nm steps) on a monocrystalline silicon substrate with demonstrated measurement accuracy better than 10 nm. A comparison of this all optical method with a mechanical scanning inspection system is discussed.

Through-focus scanning optical microscopy (TSOM) considering optical aberrations: practical implementation

Through-focus scanning optical microscopy (TSOM) method based on use of a library, which is composed of simulated defocused images of nanosized silicon lines on the top of a monocrystalline silicon substrate, is demonstrated. The images are simulated using Finite-Differences in Time-Domain (FDTD) method taking into account optical aberrations of the experimental setup, which are measured experimentally. Consideration of the optical aberrations allows us to reduce the discrepancy between experimental and simulated defocused images of the samples under study to the value of ≈2%in contrast to ≈10% when the aberrations are not taken into account. It results in ≈5% recognition accuracy for critical dimension (CD) values in the range 40-150 nm.

Self-injection locking of a laser diode to a high-Q WGM microresonator

We present the analysis of the self-injection locking of a single-frequency laser diode to a high-Q whispering gallery mode (WGM) microresonator with Rayleigh backscattering. Simple analytical formulas for the width of the locking band and resulting laser linewidth are derived.

Mechanical-free optical technology for nanostructures inspection

We present a novel all optical technology for precision nanoscale pattern inspection. The approach utilizes imaging system with the high value of axial chromatic aberration and a low-cost light source tunable in the ~30 nm wavelength bandwidth. Such combination allows us to capture defocused images in highly stable conditions without mechanical scanning of either tested sample or image sensor. Further processing of the diffraction images in the defocused planes gives one an ability to compare inspected objects and, using a library of preliminary measured data, define their geometrical parameters with nanoscale accuracy. The proposed method was tested with calibrated lines (height 50 nm, length 100 μm, width range 40-150 nm with 10 nm step) on top of monocrystalline silicon substrate. Measurement accuracy of the optical technology was estimated as ~1 nm.

Narrow-linewidth lasing and soliton Kerr microcombs with ordinary laser diodes

Narrow-linewidth lasers and optical frequency combs generated with mode-locked lasers have revolutionized optical frequency metrology. The advent of soliton Kerr frequency combs in compact crystalline or integrated ring optical microresonators has opened new horizons in academic research and industrial applications. These combs, as was naturally assumed, however, require narrow-linewidth, single-frequency pump lasers. We demonstrate that an ordinary cost-effective broadband Fabry–Pérot laser diode at the hundreds of milliwatts level, self-injection-locked to a microresonator, can be efficiently transformed to a powerful single-frequency, ultra-narrow-linewidth light source with further transformation to a coherent soliton comb oscillator. Our findings pave the way to the most compact and inexpensive highly coherent lasers, frequency comb sources, and comb-based devices for mass production.A broadband multi-frequency Fabry–Pérot laser diode, when coupled to a high-Q microresonator, can be efficiently transformed to an ~100 mW narrow-linewidth single-frequency light source, and subsequently, to a coherent soliton Kerr comb oscillator.

Kerr soliton combs in crystalline microresonators pumped by regular multifrequency diode lasers

Kerr frequency comb generated in ultra-high Q whispering gallery mode (WGM) microresonators is a promising light source for ultra-compact photonic devices due to its potential advantages of low power consumption and possibility of chip integration. We introduce a technique to stabilize and control effective generation of dissipative Kerr solitons (DKS) in nonlinear crystalline microresonators using regular commercial broad spectrum multi-frequency CW laser diodes (1550 nm; P ∼ 200 mW), self-injection locked to magnesium fluoride microresonators (Q > 109).

Kerr soliton combs with regular multifrequency diode lasers

Kerr optical frequency combs in high-Q microresonators [1] are attracting growing interest [23], especially after mode-locking via dissipative Kerr solitons (DKS) has been demonstrated on a variety of platforms [3, 4]. Such combs are a promising source for compact applications due to its potential advantages of low power consumption and possibility of chip integration. A traditional approach to obtaining DKS in microresonators relies on narrow-linewidth tunable lasers for pumping. Independently the same type microresonators could be used for significant line narrowing of diode lasers exploiting resonant Rayleigh backscattering [5] for self-injection locking [6]. Kerr soliton frequency combs have also been demonstrated with self-injection locked diode lasers [7]. Previously for self-injection locking only single frequency stabilized diode lasers were used with either Bragg-grating [6] or distributed feedback configuration [7], having narrow linewidth comparable to the resonance linewidth of the high-Q microresonator. Surprisingly, we found that the initial stabilization is not required for soliton comb generation, and simpler but more powerful diode lasers may be used, and demonstrate a technique to stabilize, generate and control coherent low-noise soliton Kerr combs using commercial broad spectrum multi-frequency CW laser diodes, self-injection-locked to an ultra-high-Q crystalline whispering-gallery-mode microresonator. In this configuration the role of the microresonator is twofold: 1) it selects and narrows the linewidth of the laser via self-injection locking, and 2) soliton Kerr comb is generated in the microresonator. We manufactured a MgF2 resonator, 5 mm in diameter with computer controlled single-point diamond turning machine and polished it with diamond slurries, achieving Q > 109. For pumping, we used free-space laser diodes (Seminex, λ∼1535, 1550 and 1650 nm, spectrum width ∼10 nm, P∼200 mW) coupled to the resonator with a total internal reflection prism. Generation of self-injection locking soliton combs stable for hours (beat note linewidth <1kHz) was observed when the laser current was adjusted [Fig. 1]. By changing current it was possible to select the pumped mode of the resonator thus gradually changing the central frequency of the soliton by 10 nm. In several cases, we observed simultaneous excitation of two solitons with different central frequencies. In this case beat note spectrum demonstrated two narrow lines separated by ∼ 10 MHz distance, corresponding to FSR difference at central frequencies. The diode multimode spectrum (10 nm) was narrowed to single mode line with FWHM of only 5 kHz, comparable to the results achieved with self-injection-locked DFB lasers.

Kerr combs in microresonators: from chaos to solitons and from theory to experiment (Conference Presentation)

Kerr frequency combs in optical passive microresonators promise new breakthrough in photonics. Such combs result from multiple hyper-parametric four-wave mixing processes. The report presents the results of recent theoretical and experimental studies, leading to the development of compact and integrated coherent frequency comb sources.