Introduction
Fourier transform ultrafast spectroscopy (FTUS) is a powerful technique that has revolutionized the way scientists capture and analyze spectra with exceptional speed and precision. By employing the principles of Fourier transform, FTUS efficiently dissects intricate signals into their frequency components by employing an interference approach with a reference signal. This method allows for the comprehensive acquisition of a spectrum, encompassing all relevant frequency information simultaneously, without scanning individual wavelengths or frequencies.
In contrast to traditional dispersive spectroscopy, which relies on prisms or diffraction gratings to measure light intensity as a function of wavelength, FTUS is not subject to the limitations of serial scanning, making it an indispensable tool across various scientific and engineering domains. The ability to acquire complete spectra rapidly and accurately sets FTUS apart, and provides advantages such as expedited data collection, high spectral resolution, and the ability to detect weak signals with high sensitivity. Its impact extends across various fields, from materials characterization to biological studies, where precise spectral analysis is crucial for scientific advancements.
A team at the Chinese Academy of Sciences in Wuhan is utilizing Moku:Pro, an advanced, FPGA-based test device that delivers more than 14 software-defined instruments — ranging from common bench necessities to unique, essential instruments — to streamline even their most experimental lab setups. Using the Lock-in Amplifier and Moku Cloud Compile, ShaoGang Yu, Ph.D., has advanced the team’s research by employing precision instrumentation in various stabilization setups.
The challenge
Despite its effectiveness, the pursuit of highly sensitive, high-resolution, and efficient FTUS technology remains a continuous endeavor for researchers. The crux of FTUS technology lies in the interferometer arm length, which is susceptible to environmental noise, including mechanical vibrations and airflow. Even minor disturbances can introduce alterations in the optical path length and interference phase, subsequently impacting signal measurement sensitivity and the signal-to-noise ratio (SNR). Thus, achieving a stable interference arm length locking mechanism is essential for advancing FTUS technology.
The current approach focuses on utilizing a reference laser that propagates coaxially with the FTUS excitation laser. By meticulously monitoring and locking the interference fringe jitter of the reference laser, researchers can attain the desired interference arm length stability. However, a significant challenge arises from the fact that the reference laser, while instrumental in stabilizing the interferometer arm length, cannot be utilized to excite the sample. As a workaround, a reference laser with a wavelength significantly different from the excitation laser is typically employed in experiments. Nevertheless, this workaround introduces a notable drawback — there is a substantial difference in phase jitter experienced by the reference laser compared to the excitation laser. This discrepancy severely constrains the enhancement of FTUS technology indicators and its broader applications, urging researchers to innovate and refine the methodology for optimal results in complex experiments and real-world applications.
The solution
To tackle this issue, Professor Yu, a researcher at the academy’s Innovation Academy for Precision Measurement Science and Technology, and his team harnessed Multi-instrument Mode for Moku:Pro in conjunction with an ultra-stable reference laser. Employing a Lock-in Amplifier, they demodulated real-time interference phase measurements and implemented custom algorithms with Moku Cloud Compile, a feature available for all Moku products that gives users access to the FPGA within the device to enable custom functionality. With Moku Cloud Compile, users simply write their desired code using a web browser, compile it in the cloud, and deploy the bitstream to one or more Moku devices in the app. For total flexibility, users can write code in HDL, start with one of our examples, or work with compatible tools like Simulink or MATLAB and HDL Coder.
This flexibility allowed Professor Yu to transform the phase in real time, enabling precise phase determination at any wavelength and facilitating seamless data acquisition and recording. This solution successfully mitigates the challenge of phase jitter induced by the notable difference in laser wavelengths, ultimately enabling the advancement of highly sensitive, high-resolution, and efficient FTUS technology.
Remarkably, this streamlined process requires only one Moku:Pro unit because it fully leverages the device’s versatile Multi-instrument Mode functionality for aspects like phase measurement, phase transformation, and comprehensive data acquisition. The development of this measurement method represents a significant milestone in the continuous improvement of FTUS technology, substantially broadening its potential applications. Its impact extends across diverse scientific fields, including physics, chemistry, biology, and astronomy research, where it holds immense value and promising prospects for further breakthroughs.
Figure 1 and Figure 2 depict the setups for the Multi-instrument Mode and Lock-in Amplifier configurations. In this system, the Lock-in Amplifier and Moku Cloud Compile work together to perform the phase transformation. Subsequently, a second Lock-in Amplifier in Slot 3 demodulates the input signal using the transformed phase signal. The resulting demodulated data is then gathered by the Data Logger in Slot 4 for processing. This research has led to a novel algorithm to advance the field of FTUS, which was implemented with Moku Cloud Compile.
Figure 1: Moku:Pro Multi-instrument Mode configuration, with four instruments deployed simultaneously.
Figure 2: FTUS Lock-in Amplifier configuration with a 500 Hz lowpass filter and an external PLL reference.
The result
After performing a fast Fourier transform (FFT) on the demodulated data presented in Figure 3, the team successfully obtained an accurate spectrum with a high SNR (shown in Figure 4). This high-precision spectrum represents the physical characteristics of the particle being analyzed.
Figure 3: Demodulation results captured using the Lock-in Amplifier’s embedded Data Logger.
In Figure 4, the blue line corresponds to data captured using a laser with a 700 nm wavelength, which is 74 nm away from the target wavelength. In contrast, the red line represents data obtained using an 800 nm wavelength laser for demodulation. Notably, the red line demonstrates a better SNR compared to the blue line. This improvement can be attributed to the fact that the wavelength of laser in the red line is 48 nm closer to the target wavelength, resulting in fewer phase disturbances in the final results.
By harnessing the phase transformation capabilities enabled by Moku:Pro, researchers can finely tune the wavelength of the demodulation signal to align it closely with the target wavelength. As a result, the SNR is further enhanced, offering significant advantages in the detection of weak signals and operation in high-noise environments. This underscores the capability and potential of Moku:Pro to advance the field of FTUS.
Figure 4: Spectrum result after fast Fourier transform (FFT).
Reflecting on his team’s research accomplishments, Professor Yu praised the performance and flexibility of Moku:Pro.
“Moku:Pro not only offers powerful software-defined instruments such as a Lock-in Amplifier, but Multi-instrument Mode and Moku Cloud Compile give us unprecedented flexibility,” he said. “The combination of the two helped us complete experiments that were very difficult in the past.”
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