Measurement of Optical Frequency Shifts
Methods and Device to Measure Small Changes to the Center Frequency of a Light Pulse
At A Glance
Researchers at Colorado State University have developed an apparatus and method for measuring small changes in the centroid of the power spectrum of a light field by conversion of optical frequency centroid shifts into time delays, which permits significantly higher sensitivity than the most sensitive Raman sensing techniques. This spectroscopy imaging technique can be used for chemical detection in nearly any industry or application that differentiates molecules or the chemical nature of compounds.
Detection of low concentrations of molecular or material impurities is needed for many scientific and industrial applications. Vibrational spectral techniques of Raman scattering and mid-infrared (MIR) absorption, as examples, are widely used for these purposes. Vibrational spectroscopy uniquely identifies molecules by recording intrinsic vibrational spectral features of the molecules. MIR imaging exhibits poor spatial resolution due to the long wavelengths, and presents challenges due to pervasive absorption and the poor quality of mid-infrared light sources and detectors. Although Raman microscopy uses visible or near infrared light, permitting high spatial resolution imaging, Raman interactions are weak, which limits Raman spectroscopic imaging to relatively high concentration levels. Doppler Raman spectroscopy is based on the measurement of small frequency shifts imparted to pulses in a short laser pulse train interacting with coherently excited Raman-active vibrational modes. That is, a short laser pump pulse is directed into a specimen containing molecules having Raman-active vibrational modes. This pulse is followed by a probe pulse that acquires a shift in the centroid of the pulse spectrum due to the coherently excited vibrations. Typical frequency shifts are tens of GHz; however detection of nanomole concentrations of molecules requires resolving shifts of about 500 Hz. Currently, the highest resolution spectrometers have a maximum resolution in the range of 1 GHz, which is orders of magnitude too course.
This innovation is a method and device for measuring very small changes in optical frequency in a chain of optical pulses. It is far more sensitive that other approaches and is applicable to frequency shifts that cannot be measured by standard interferometric approaches. The technology is widely applicable and is well suited for detecting any frequency shift that is the same pulse to pulse.
The key to the technology is to convert the optical frequency shift to a time delay, which is easily measured electronically. Although conceptually simple, this technique is powerful and can dramatically increase instrument sensitivity. For example, direct measurement via a grating spectrometer is limited by spectrometer resolution to about 1 GHz. With this technology, frequency shifts of just 1 kHz may be detected, representing an improvement of 6 orders of magnitude.
This innovation was originally conceived for application to Raman spectroscopy and, for this use, offers the exciting possibility of improving Raman sensitivities to the previously unattainable level of single molecule detection.
- Method and device for measuring very small frequency shifts in an optical pulse.
- This type of frequency shift cannot be measured using standard interferometry.
- Applicable to any frequency shift that is the same pulse to pulse.
- For Raman spectroscopy, this technology should lead to orders of magnitude sensitivity improvement (“single molecule detection”).
- Biomedical diagnostics
- Polymer manufacturing
- Trace chemical detection
Last updated: April 2020