Professional Knowledge

Laser linewidth measurement

Although both spectrum and frequency spectrum are electromagnetic spectrum, due to the difference in frequency, the analysis methods and test instruments of spectrum and frequency spectrum are very different. Some problems are difficult to solve in the optical domain, but it is easier to solve them through frequency conversion to the electrical domain.
For example, spectrometers that use scanning diffraction gratings as frequency-selective filters are currently the most widely used in commercial spectrometers. They have a wide wavelength scanning range (1 μm) and a large dynamic range (above 60dB), but the wavelength resolution is only limited to a dozen. A picometer (>1GHz) or so. It is impossible to use such an instrument to directly measure the laser spectrum with a linewidth in the order of megahertz. At present, the linewidth of DFB and DBR semiconductor lasers is in the order of 10MHz, and after the use of external cavity technology to greatly narrow the spectral linewidth, the linewidth of fiber lasers can already be lower than the kilohertz order. To further improve the resolution bandwidth of the spectrometer, it is very difficult to achieve extremely narrow linewidth laser spectroscopy. However, this problem can be easily solved by optical heterodyne.
At present, both Agilent and R&S companies have spectrum analyzers with a resolution bandwidth of 10 Hz. The real-time spectrum analyzer can also increase the resolution to 0.1MHz. In theory, the use of optical heterodyne technology can solve the problem of measurement and analysis of millihertz linewidth laser spectroscopy. Review the development history of optical heterodyne spectrum analysis technology, whether it is the dual-beam optical heterodyne method of DFB lasers or the time-delayed white heterodyne method of single tunable lasers, the precise measurement of the narrow spectral linewidth is achieved through spectrum analysis . Using optical heterodyne technology to move the spectrum of the optical domain to the easy-to-handle intermediate frequency electrical domain, the resolution of the electrical domain spectrometer can easily reach the order of kilohertz or even hertz. For high-frequency spectrum analyzers, the highest resolution has reached 0.1 mHz. Therefore, it is easy to solve the problem of measurement and analysis of narrow linewidth laser spectroscopy, which is a problem that cannot be solved by direct spectroscopy analysis. Makes the accuracy of spectral analysis greatly improved.
Applications of narrow linewidth lasers:
      1. Oil pipeline optical fiber sensing
      2. Acoustic sensors, hydrophones
      3. Lidar, ranging, remote sensing
      4. Coherent optical communication
      5. Laser spectroscopy, atmospheric absorption measurement
      6. Laser seed source
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