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Ultrafast electro-optic dual-comb multi-point vibrometer for microscopy applications

Final Report Summary - FAST-TEMPO (Ultrafast electro-optic dual-comb multi-point vibrometer for microscopy applications)

A frequency comb is a light source whose spectrum is composed of a set of laser lines that appear evenly spaced in frequency. The position of these lines can be linked to a microwave or optical reference with an outstanding degree of accuracy, thus enabling different applications in precision frequency synthesis and metrology [1]. A particularly relevant application of laser frequency combs is in molecular spectroscopy [2]. Here, the technique known as dual-comb spectroscopy [3] allows for measuring the response of a spectroscopic sample over a very broad bandwidth using two laser frequency combs. The advantage of dual-comb spectroscopy over other spectroscopy modalities (such as Fourier transform spectroscopy) is its ability to resolve the individual frequency lines of the comb, thus allowing a 4-6 orders of magnitude improvement in accuracy and resolution [4].

There are in the literature several different implementations of dual-comb spectrometers (aka dual-comb interferometers). Electro-optic dual-comb interferometers [5-8] are an emerging modality of dual-comb systems, where a single continuous-wave laser feeds simultaneously two electro-optic comb generators. These systems are built in a very robust manner [9] and can be operated without complex stabilization mechanisms [5]. In addition, electro-optic dual-comb interferometers can operate at a very high acquisition speed at the expense of spectral sampling resolution.

In the framework of this project, we demonstrated an electro-optic dual-comb spectrometer that operates at 25 MHz refresh rate and can measure a 1 THz bandwidth in the telecommunications short-wave infrared region of the electromagnetic spectrum [7]. The system leverages the latest advances in fiber-optic communication hardware [10] and opens up possibilities for realizing precision frequency metrology in the sub-microsecond regime. Although the speed was certainly impressive, the bandwidth was limited. There is a certain type of applications for dual-comb spectrometers in nonlinear microscopy that require to operate over a much broader bandwidth [11]. Consequently, the next effort on the project was to extend the bandwidth of our system over 4 THz. This required using a different acquisition system and utilizing a specialty fiber to extend the bandwidth of the electro-optic comb generators [12]. Although the bandwidth of the system was quadrupled, we observed a degradation of the signal to noise ratio per spectral line. This meant in practice that we had to decrease the speed of the system in order to accumulate sufficient signal power. The next step was to explore metrological applications that could leverage the speed of the system but did not place strong demands with regards to the bandwidth. We concentrated the efforts in high-speed vibrometry/reflectometry applications, in line with the project’s initial aim. We re-assembled the system presented in [7] and set up a proof of concept utilizing an ultrasonic speaker as the dynamic reflective sample. We demonstrated that our electro-optic dual-comb system is capable to retrieve nanometric movements of dynamic objects up to 250 kHz. The amplitude’s performance is orders of magnitude better than other comb-based vibrometric systems that are commercially available [13].

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