Multichannel Lock-in Detection Research Articles

Broadband stimulated Raman imaging based on multi-channel lock-in detection for spectral histopathology

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.

Simultaneous Broadband Vector Magnetometry Using Solid-State Spins

We demonstrate a vector magnetometer that simultaneously measures all Cartesian components of a dynamic magnetic field using an ensemble of nitrogen-vacancy (NV) centers in a single-crystal diamond. Optical NV-diamond measurements provide high-sensitivity, broadband magnetometry under ambient or extreme physical conditions; and the fixed crystallographic axes inherent to this solid-state system enable vector sensing free from heading errors. In the present device, multi-channel lock-in detection extracts the magnetic-field-dependent spin resonance shifts of NVs oriented along all four tetrahedral diamond axes from the optical signal measured on a single detector. The sensor operates from near DC up to a $12.5$ kHz measurement bandwidth; and simultaneously achieves $\sim\!50$ pT/$\sqrt{\text{Hz}}$ magnetic field sensitivity for each Cartesian component, which is to date the highest demonstrated sensitivity of a full vector magnetometer employing solid-state spins. Compared to optimized devices interrogating the four NV orientations sequentially, the simultaneous vector magnetometer enables a $4\times$ measurement speedup. This technique can be extended to pulsed-type sensing protocols and parallel wide-field magnetic imaging.

Stochastic multi-channel lock-in detection

High-precision measurements benefit from lock-in detection of small signals. Here we discuss the extension of lock-in detection to many channels, using mutually orthogonal modulation waveforms, and show how the choice of waveforms affects the information content of the signal. We also consider how well the detection scheme rejects noise, both random and correlated. We address the particular difficulty of rejecting a background drift that makes a reproducible offset in the output signal and we show how a systematic error can be avoided by changing the waveforms between runs and averaging over many runs. These advances made possible a recent measurement of the electron's electric dipole moment (Hudson et al 2011 Nature 473 493).

Development of a multiplex stimulated Raman microscope for spectral imaging through multi-channel lock-in detection

We report the development of a multiplex stimulated Raman microscope for spectral imaging through multi-channel lock-in detection with a single light source. A white pump beam is prepared with a piece of photonic crystal fiber (PCF). The system does not require the synchronization of plural light sources or the scanning of their wavelengths, and thus a jitter-free pair of pump and Stokes beams is obtained, and a high degree of temporal synchronization is attained in the spectra. The multi-channel lock-in detection (extended to 128 channels) enables the observation of pseudo-continuous stimulated Raman spectra, demonstrating the strong ability of qualitative analysis to identify various types of C-H stretching modes such as the symmetric and asymmetric modes of the methylene∕methyl and aromatic groups. Images of a mixed film of polystyrene and polymethylmethacrylate are presented to demonstrate the system's spectral imaging ability. The spatial distribution of these materials is successfully captured through one-time imaging, although the noise of the white light pump beam generated with the PCF limits the system's imaging speed.

Optical imaging of high-frequency focused ultrasonic field using a Nomarski interferometer coupled with multichannel lock-in detection

We present a nondestructive optical technique for quantitatively imaging a focused ultrasonic field in continuous-wave mode within clear media. We used a Nomarski interferometer configuration coupled with a couple charge device array and multichannel lock-in detection. A two-dimensional full-field image of pressure amplitude can be obtained in only one acquisition. This method was experimentally demonstrated with a high-frequency ultrasonic transducer and experimental measurements of acoustic pressure obtained at 27 MHz. This technique is very interesting because it does not introduce any disturbance in the ultrasonic field and does not need any scanning mechanisms to acquire images. It can be useful for probing high-frequency acoustic fields and for calibrating HF-piezo-electric transducers.

Online fluorescence suppression in modulated Raman spectroscopy.

Label-free chemical characterization of single cells is an important aim for biomedical research. Standard Raman spectroscopy provides intrinsic biochemical markers for noninvasive analysis of biological samples but is often hindered by the presence of fluorescence background. In this paper, we present an innovative modulated Raman spectroscopy technique to filter out the Raman spectra from the fluorescence background. The method is based on the principle that the fluorescence background does not change whereas the Raman scattering is shifted by the periodical modulation of the laser wavelength. Exploiting this physical property and importantly the multichannel lock-in detection of the Raman signal, the modulation technique fulfills the requirements of an effective fluorescence subtraction method. Indeed, once the synchronization and calibration procedure is performed, minimal user intervention is required, making the method online and less time-consuming than the other fluorescent suppression methods. We analyze the modulated Raman signal and shifted excitation Raman difference spectroscopy (SERDS) signal of 2 mum-sized polystyrene beads suspended in a solution of fluorescent dye as a function of modulation rate. We show that the signal-to-noise ratio of the modulated Raman spectra at the highest modulation rate is 3 times higher than the SERDS one. To finally evaluate the real benefits of the modulated Raman spectroscopy, we apply our technique to Chinese hamster ovary cells (CHO). Specifically, by analyzing separate spectra from the membrane, cytoplasm, and nucleus of CHO cells, we demonstrate the ability of this method to obtain localized sensitive chemical information from cells, away from the interfering fluorescence background. In particular, statistical analysis of the Raman data and classification using PCA (principal component analysis) indicate that our method allows us to distinguish between different cell locations with higher sensitivity and specificity, avoiding potential misinterpretation of the data obtained using standard background procedures.

Quantitative Thermal Imaging with CCD Array Coupled to an Heterodyne Multichannel Lock-in Detection

Quantitative Thermal Imaging with CCD Array Coupled to an Heterodyne Multichannel Lock-in Detection

Electrolytic flux imaging on band electrodes by interferometric microscopy

In this paper we present mass transfer mapping in solution at the micron scale using interferometric microscopy. In its present form, the technique is sensitive to the solution refractive index component parallel to the surface of the electrode. The technique is able to detect optical path differences of the order of the picometer. We show the feasibility and performances of our set-up, an interferometric microscope equipped with a CCD camera and a multichannel lock-in detection, by imaging ionic fluxes in solution between band electrodes. The experiments are performed with the usual electrochemical conditions on the following systems: Fe 3+/Fe 2+ in sulphuric acid, Fe(CN) 6 3−/Fe(CN) 6 4− in KCl aqueous solution and Ce 4+/Ce 3+ in nitric acid. The electrolytic flux imaging is analyzed along transverse and longitudinal profiles of the concentration gradients in relation with the current flowing through the cell.

Roughness measurements in the picometric range using a polarization interferometer and a multichannel lockin detection technique

Roughness measurements in the picometric range using a polarization interferometer and a multichannel lockin detection technique

Multichannel Nomarski microscope with polarization modulation: performance and applications

Using a two-dimensional CCD array, a photoelastic modulator, and a commercial Nomarski microscope, we built a differential polarization interferometer. Multichannel lock-in detection of the signal allowed us to reach a shot-noise limit corresponding to 5-pm path differences for a few seconds of recording time. This interferometer was used as a surface profilometer as well as a system for imaging through scattering media.