Avinash’s Blog

Post 2 (05/07/2021)

 Integrated Frequency Combs

The project aims to generate new frequencies from visible to mid-IR spectrum and utilizing them for high precision spectroscopy through comb generation.  At AMO, we are developing the above-mentioned technology on a well-established & sustainable CMOS platform.

Considering CMOS compatible integrated photonics platforms, Si (Silicon) and Si3N4 (Silicon Nitride) have been the way to go as shown in the research over the years. In this project, we are using Si3N4 for having negligible loss at the laser source (1550 nm/ 193.4 THz) in use.

The exploited Si3N4 property here is the high third-order optical nonlinearity, which results in comb generation through four wave mixing (parametric nonlinear process). Various designs for comb generation through Si3N4 have been already proposed based on different figures of merit. In this project, we are focusing on ring shaped resonators. These resonators are an ideal testbed to enhance the optical nonlinearity of the photonic platform.

We used device and circuit level photonic simulations for the device design optimized for comb generation at source wavelength of 1550 nm (Figure 1).

Figure 1: Schematic diagram of the resonator device, top view (left side) & cross section (right side).


Nonlinear simulations for comb generation were done in collaboration with partners from Sapienza University of Rome. To describe the comb generation, we used the Lugiato-Lefever equation (LLE) with a continuous laser pump at 1550 nm in the normal dispersion regime. We firstly verified the resultant code by reproducing the already published results based on the same formulation of LLE and then used it with the device parameters under consideration for fabrication. With the help of the above simulations, we were able to understand the dynamics of comb formation in our device (theoretically) and estimated that we can reach the threshold power needed for comb generation on our on-chip resonators.

Now, the device fabrication is on the way. Check out the next MOCCA newsletter for more!!

For more information on the project, please visit: https://mocca.astonphotonics.uk/

For more information on AMO’s research, please visit: https://www.amo.de/research/photonics/

Post 1

Towards Advanced Optical Frequency Combs


In last 20 years or so, extraordinary technological advancements led to the fundamental change in lifestyle of human beings and driving strongly towards the onset of Fourth Industrial revolution. The main factors that are playing very key important role in this age of industrial revolution are the rate of data transmission and time accuracy.

Frequency combs were also developed during this period of time and boomed considering its applications in the technology and data driven industries.

To put things into perspective,

‘‘Since Galileo Galilei and Christiaan Huygens invented the pendulum clock, time and frequency have been the quantities that we can measure with the highest precision.

Today, it is often a good strategy to transform other quantities such as length or voltage into a frequency in order to make accurate measurement. With better measuring tools, one can look where no one has looked before. More than once, seemingly minute differences between measurement and theory have led to major advances in fundamental knowledge. The birth of modern science itself is intimately linked to the art of accurate measurements.

In 1967, the Conference Generale des Poids et Mesures (CGPM) has defined the second, our unit of time, as the period during which a cesium-133 atom oscillates 9 192 631 770 times on a hyperfine clock transition in the atomic ground state. By slicing time into a hundred thousand times finer intervals, such clocks will greatly extend the frontiers of time and frequency metrology. The long missing clockwork mechanism can now be realized with optical frequency combs (OFCs), an ultraprecise measuring tool that can link and compare optical frequencies and microwave frequencies phase coherently in a single step.

Optical Frequency comb provide powerful tools for new tests of fundamental physics laws. Precise comparisons of optical resonance frequencies of atomic hydrogen and other atoms with the microwave frequency of a cesium atomic clocks are already establishing sensitive limits for possible slow variations of fundamental constants.’’

  • Theodor W. Hänsch (Nobel Lecture 2006: Passion for precision)


Optical high harmonic generation and new device platforms are extending frequency combs into the extreme ultraviolet, opening a new spectral territory to precision laser spectroscopy, GPS technology and LIDARs.

Courtesy: NIST


The figure above shows spectroscopic techniques to analyze the frequency combs after the light interacts with a sample. Frequency combs can be used to analyze different materials using the same old spectroscopic architecture but with high precision and more efficiently


The remarkable technical capabilities outlined above gained John “Jan” Hall and Theodor Hänsch recognition by the Nobel Committee in 2005 for their life long contributions to the field of precision optical frequency metrology, as well as for their technical vision and expertise that resulted in the realization of the optical frequency combs (OFC) with mode locked lasers (MLLs).

Professor Dr. Theodor W. Hänsch founded Menlo systems in 2001 which is a leading developer and global supplier of instrumentation for precision metrology based on developed technology of MLLs: optical frequency combs.

Please check out their promotional video on Laser based optical frequency combs:

Courtsey: Menlo Systems


Paradigm Shift:

‘‘The past 15 years have also seen the development of chip-scale systems based on microresonators and semiconductor systems. The compact size of these systems yield great excitement about the possibility for chip-scale and photonically integrated OFC sources. OFCs based on microresonators, or micro-combs, differ significantly in operation from MLLs because they are not lasers, but low-loss, optical resonators. The first of these systems developed as OFCs were based on suspended silica micro-toroids.

Micro-resonators act as build up cavities that enable high-nonlinearity over long storage times, or equivalently long-interaction lengths, in very much the same way as do nonlinear fibers. Via degenerate- and non-degenerate four-wave mixing, a resonantly coupled single-frequency pump source is converted to a comb of optical frequencies.

Although compact sources based on microresonator and semiconductor systems still face challenges to full optical integration, to date these platforms offer the only possible architectures for chip-based and integrated comb systems. The future integration of compact OFCs with CMOS compatible photonic waveguides based on, for example lithium niobate, silicon or silicon nitride, diode lasers and miniature optical clocks might one day enable sub-watt systems for both optical and microwave synthesis, enabling cost-efficient production for dissemination of OFC sources and products to larger commercial markets.’’

Fortier, T., Baumann, E. 20 years of developments in optical frequency comb technology and applications. Commun Phys 2, 153 (2019)

Courtesy: Gaeta, A.L., Lipson, M. & Kippenberg, T.J. Photonic-chip-based frequency combs. Nature Photon 13, 158–169 (2019)

Paradigm Diversification: Advanced Frequency Combs

Recent burst in the applications of optical frequency combs has led to development of different frequency comb generation techniques. Researchers all around studying different mechanisms for generating frequency combs efficiently using high harmonic generations, scalable fabrication methods with different materials.

MOCCA (Multiscale Optical Frequency Combs: Advanced Technologies and Applications) is a European Industrial Doctorate programme aimed for research in developing new technologies and methods supporting the development of advanced frequency combs. The MOCCA consortium consists of highly prestigious universities & companies around Europe.

Silicon (Si) / Silicon Nitride (SiN) photonic platform to generate frequency combs with very compact and cost efficient integrated photonic circuits. Si & SiN are CMOS compatible materials and are heavily used in the area of microelectronics.

This research will be benefited from the existing CMOS infrastructure in terms of cost per final device and will help in boosting the application of Si in the area of integrated photonic devices.

For more information on the project, please visit: https://mocca.astonphotonics.uk/

For more information on AMO’s research, please visit:  https://www.amo.de/research/photonics/