Spontaneous Raman microscopy reveals the chemical composition of a sample in a label-free and non-invasive fashion by directly measuring the vibrational spectra of molecules. However, its extremely low cross section prevents its application to fast imaging. Stimulated Raman scattering (SRS) amplifies the signal by several orders of magnitude thanks to the coherent nature of the nonlinear process, thus unlocking high-speed microscopy applications that provide analytical information to elucidate biochemical mechanisms with subcellular resolution. Nevertheless, in its standard implementation, narrowband SRS provides images at only one frequency at a time, which is not sufficient to distinguish constituents with overlapping Raman bands. Here, we report a broadband SRS microscope equipped with a home-built multichannel lock-in amplifier simultaneously measuring the SRS signal at 32 frequencies with integration time down to 44 µs, allowing for detailed, high spatial resolution mapping of spectrally congested samples. We demonstrate the capability of our microscope to differentiate the chemical constituents of heterogeneous samples by measuring the relative concentrations of different fatty acids in cultured hepatocytes at the single lipid droplet level and by differentiating tumor from peritumoral tissue in a preclinical mouse model of fibrosarcoma.

We introduce a multi-channel integrated circuit for fast Stimulated Raman Scattering (SRS) microscopy on multiple simultaneous frequencies at a frame rate higher than 1 frame/s. The chip is a 4-channel differential readout system, based on the lock-in technique. It is able to measure down to 10 ppm SRS signal, over a wide range of input optical powers (50 μW -600 μW per channel), with a pixel dwell time of only 30 μs. Each acquisition channel includes 2 low-noise preamplifiers, 2 variablegain amplifiers, a fully differential voltage subtractor and a lockin demodulator. The differential readout electronics rejects the power fluctuations of the laser and it is automatically balanced by an analog feedback loop over ± 30 % input power mismatches. Thanks to the autobalancing network, the pixel dwell time is reduced by a factor up to 225 with a settling time of only 10 μs. The chip is fabricated in AMS 0.35 μm CMOS technology and it is included in a combined electronics and optical system. Both single-pixel spectral measurements and multi-spectral imaging measurements are presented to validate the full SRS microscope.

The intrinsic sensitivity limit of Stimulated Raman Spectroscopy (SRS) is given by the shot noise of the optical stimulation. However, it is seldom reached due to the electronic noise of the front-end amplifier and the intensity fluctuations of the laser source. Here, we present a low-noise differential amplifier able to compensate the common-mode fluctuations given by the laser and to reach a sensitivity better than 10 ppm thanks to the lock-in technique.

In Broadband Stimulated Raman Spectroscopy, the intrinsic limit given by the laser shot noise is seldom reached due to the electronic noise of the front-end amplifier and the intensity fluctuations of the laser source. In this paper we present a low-noise multi-channel acquisition system, with an integration-oriented design, able to compensate the common-mode fluctuations of the laser output power with the pseudo-differential structure and reach a sensitivity better than 10 ppm thanks to the lock-in technique.

The ultimate resolution limit of the current frontend of wideband impedance detection circuits is set by the input total capacitance and input equivalent noise. We present a solution to reduce the noise of more than an order of magnitude when the capacitance cannot be further minimized. By introducing a properly-chosen inductor (and a proper and stable biasing network) it is possible to cancel the capacitive reactance at the resonance frequency (~10 MHz) which becomes the sensing frequency, thus significantly reducing the detection noise, while preserving the accuracy of the transfer function. A detailed analysis of the scheme, and its experimental validation with different inductors are presented. In all cases an improvement larger than 10 is achieved with an input-referred current spectral density of ~1 pA/√Hz at the operating frequency of ~20 MHz.