I. What is Laser Linewidth Testing?
Laser linewidth refers to the spectral width of a laser signal on the frequency or wavelength scale, typically evaluated by the Full Width at Half Maximum (FWHM) of the optical spectrum. In simple terms, it measures the "purity" of the laser spectrum – the narrower the linewidth, the better the monochromaticity and the purer the frequency.
For narrow‑linewidth lasers, the industry generally classifies those with a linewidth of less than 50 kHz as narrow‑linewidth lasers. The essence of linewidth testing is to obtain this spectral width value through precision measurement, in order to evaluate the frequency stability and coherence characteristics of the laser.
II. Laser Linewidth Testing Standards
Laser linewidth testing mainly follows the following standards and specifications:
T/G1 XXX—2022 (Testing specification for narrow‑linewidth lasers): Issued by the China International Science and Technology Promotion Association, this standard specifies methods for determining the main parameters of narrow‑linewidth lasers, including terminology definitions, test conditions, test methods, etc.
JJF (Electronics) 30903‑2006 (Calibration specification for single‑longitudinal‑mode laser linewidth): Applicable to the calibration of single‑longitudinal‑mode laser linewidth and the calibration of test systems.
International standard ISO 13142:2015: Specifies interferometric methods for laser linewidth measurement.
III. Laser Linewidth Testing Methods
The current mainstream method for linewidth testing is the delayed self‑heterodyne interferometry method (also known as the delayed self‑homodyne method). Its basic principle is: the laser under test is split into two paths – one path passes through a long optical fiber delay line, and the other path is frequency‑shifted by an acousto‑optic modulator (AOM). After recombination, the two beams enter a photodetector to generate a beat frequency signal. The power spectrum of the beat frequency signal is analysed using a spectrum analyser, and its FWHM is the laser linewidth.
Core test setup: Mach‑Zehnder fiber interferometer (including a 3 dB coupler), optical fiber delay line (the delay time must be greater than three times the coherence time of the laser under test), photodetector, and spectrum analyser.
Key test conditions: Ambient temperature 15 °C to 35 °C, humidity 45 %RH to 75 %RH, with no significant airflow or vibration interference. The laser under test must be stabilised for more than 30 minutes before testing. The measurement resolution is determined by both the fiber delay time and the spectrum analyser resolution, and it must be ensured that the system resolution is better than the linewidth of the laser under test.
IV. Laser Linewidth Testing Process
Test preparation: Select compliant test instruments (metrologically calibrated) and set up the delayed self‑heterodyne interferometric test system in accordance with standard specifications.
Environmental verification: Verify that the test environment conditions (temperature, humidity, vibration, etc.) meet the specified requirements.
Laser startup: Start the laser under test and wait for it to reach a stable operating state (generally more than 30 minutes). Adjust the variable attenuator to ensure the input power is appropriate (generally less than 20 mW, to avoid linewidth broadening caused by fiber nonlinear effects).
Data acquisition: Feed the beat frequency signal from the interferometer output into the photodetector, then use the spectrum analyser to record the electrical signal spectrum. If necessary, perform multiple averaging of the measurement results to reduce errors.
Linewidth calculation: Read the FWHM of the power spectrum on the spectrum analyser – this value is the linewidth of the laser under test.
Result reporting: Compile the test data and issue a test report.
V. Significance of Laser Linewidth Testing
Linewidth is one of the core indicators for evaluating the performance of single‑frequency lasers. Accurate measurement of laser linewidth is crucial for advanced applications such as coherent optical communications, fiber optic sensing, quantum computing, and precision metrology. For laser R&D and production, linewidth testing helps optimise laser design, control drive noise, and ensure product quality consistency.
VI. Where to Find a Laser Linewidth Testing Body
Shenzhen Zhongwei Inspection Technology Co., Ltd. is a professional laser equipment testing and certification organisation, with more than 70 laser‑authorised standards and over 120 laser testing capability parameters. Its measurement power range covers pW to 5 kW, and pulse measurement is capable down to the picosecond level. The company is equipped with fully imported precision instruments, and its engineering team has been engaged in laser equipment testing and certification for over ten years, providing one‑stop services from laser performance parameter testing to laser eye safety certification.
Its service scope includes: industrial, medical, aesthetic, measurement, and consumer lasers, as well as LiDAR and other products – covering laser class testing and assessment, laser safety classification certification (IEC 60825‑1), US FDA registration and certification, and laser beam quality analysis. Whether you need laser linewidth testing or export compliance certification for laser products, Zhongwei Inspection can provide professional and reliable technical support.
