Laser linewidth is the core metric for evaluating the spectral purity of a laser source. A narrower linewidth corresponds to superior monochromaticity and purer frequency output. In the industry, lasers with a linewidth below 50 kHz are generally defined as narrow linewidth lasers.
Purpose of Linewidth Testing
Precision measurement is adopted to obtain the spectral linewidth value and evaluate the laser’s frequency stability and coherent properties. This measurement is critical for applications including fiber-optic communication, precision metrology and coherent detection.
1. Standards Governing Laser Linewidth Testing
Laser linewidth measurement complies with the following specifications:
T/G1 XXX-2022 Test Specification for Narrow Linewidth Lasers
Issued by the China International Council for the Promotion of Science and Technology, it defines measurement methods for key parameters of narrow linewidth lasers, covering terminology definitions, test environments and measurement procedures.
JJF (Electronics) 30903-2006 Calibration Specification for Linewidth of Single Longitudinal Mode Lasers
Applicable to linewidth calibration of single longitudinal mode lasers and calibration of measurement systems.
ISO 13142:2015 International Standard
Specifies the interferometric method for laser linewidth measurement.
2. Primary Measurement Technique: Delayed Self-Heterodyne Interferometry
Delayed self-heterodyne interferometry is the dominant mainstream method for laser linewidth testing.
Basic Principle
The laser beam under test is split into two optical paths. One path passes through a long fiber delay line to generate optical delay, while the other is frequency-shifted via an Acousto-Optic Modulator (AOM). The two beams are recombined and injected into a photodetector to generate a beat signal. A spectrum analyzer analyzes the power spectrum of the beat signal, and its Full Width at Half Maximum (FWHM) equals the laser linewidth.
Core Test Equipment
Mach-Zehnder fiber interferometer equipped with 3dB coupler, fiber delay line (delay time no less than three times the coherence time of the laser under test), photodetector and spectrum analyzer.
Mandatory Environmental Conditions
Ambient temperature: 15°C to 35°C
Relative humidity: 45%RH to 75%RH
No prominent air flow or vibration interference
The laser under test shall operate stably for more than 30 minutes before testing.
3. Six-Step Standard Linewidth Measurement Procedure
Step 1 System Assembly and Inspection
Deploy calibrated, verified test instruments to build the delayed self-heterodyne interferometry system, and verify all optical and electrical connections are stable and intact.
Step 2 Environmental Verification
Confirm temperature, humidity, vibration and other environmental factors fully comply with standard requirements.
Step 3 Laser Warm-Up
Power on the laser source and allow it to stabilize for a minimum of 30 minutes. Adjust the variable optical attenuator to control input power below 20 mW to prevent linewidth broadening induced by fiber nonlinear effects.
Step 4 Data Acquisition
Connect the beat signal output from the interferometer to the photodetector, and record the electrical spectrum via the spectrum analyzer. Multiple averaging cycles may be applied to reduce measurement errors when necessary.
Step 5 Linewidth Calculation
Read the FWHM value of the power spectrum displayed on the spectrum analyzer, which is the final linewidth of the tested laser.
Step 6 Test Report Generation
Sort and organize all raw test data and issue an official test report.
4. Application Scenarios of Linewidth Measurement
Linewidth serves as one of the key performance indicators for single-frequency lasers. Accurate linewidth measurement is indispensable across the following high-end sectors:
Coherent optical communication: Narrow linewidth is the fundamental prerequisite for effective coherence between local oscillator light and signal light
Fiber-optic sensing: Linewidth determines maximum detection distance and measurement precision
Quantum computing: Ultra-narrow linewidth lasers are essential hardware for ion trap quantum computing systems
Precision metrology: High-precision equipment such as gravitational wave detectors and optical clocks relies on narrow linewidth laser sources
5. Our Professional Laser Testing Services
As a professional third-party laser testing laboratory, we provide comprehensive technical support as follows:
Full-parameter laser testing: Linewidth, wavelength, output power, beam quality, pulse characteristics and other optical performance metrics
Comprehensive testing capacity: We are accredited for more than 70 laser standards and capable of measuring over 120 distinct parameters. Our power testing range covers pW up to 5 kW, with picosecond-level pulse measurement capability.
Authorized laboratory credentials: CNAS and CMA accredited lab with internationally mutually recognized test reports
Extended one-stop compliance services: Laser safety classification certification to IEC 60825-1, US FDA laser product registration, laser beam quality characterization and more
