In the field of laser technology, most traditional lasers can only output laser light of a single or a few fixed wavelengths. However, scientific research and industrial applications often require laser sources that can be continuously and precisely tuned within a certain spectral range. Tunable lasers have emerged as the times require. Like an adjustable "optical ruler" or "color pen", they provide unprecedented flexibility for us to explore material properties, transmit information, and conduct precision measurements.

1. The principle of wavelength tuning 
Wavelength-tunable lasers actively and controllably change the central wavelength of the laser output through a specific physical mechanism. Their basic principle is mainly based on the three conditions for laser generation: stimulated emission, population inversion, and resonant feedback. The essence of tuning is to modify the laser’s resonant cavity conditions or the gain spectrum of the gain medium.

Principle diagram of a tunable laser

2. The Main Technical Approaches for Wavelength Tuning 

2.1 Controlling Resonator Losses 
This is the most commonly used method. A wavelength-selective element (such as a grating, prism, or Fabry-Perot etalon) is introduced into the laser resonator cavity. By physically rotating or electrically adjusting this element, only light of a specific wavelength meets the oscillation condition inside the cavity—meaning it experiences minimal loss—while light at other wavelengths is suppressed, thereby enabling precise wavelength selection and tuning.

2.2 Adjusting the Gain Medium 
Certain gain media (such as dyes and diodes) inherently have a broad gain spectrum. By changing the pumping conditions, temperature, or applying an external field (e.g., electric current, magnetic field), the central position or shape of their gain spectrum can be altered, thereby achieving tuning of the output wavelength.

2.3 Nonlinear Frequency Transformation 
Using nonlinear optical crystals (such as PPLN, periodically poled lithium niobate), lasers with fixed wavelengths can be converted into new wavelengths (e.g., frequency doubling, optical parametric oscillation). By adjusting the crystal's temperature, angle, or polarization period, a wide range of tuning of the output wavelength can be achieved.

3. Classification of Wavelength-Tunable Lasers 
Based on their structure and tuning mechanisms, wavelength-tunable lasers can be classified into the following major categories:

3.1 Distributed Feedback Lasers and Distributed Bragg Reflector Lasers 
A Bragg grating is fabricated in or near the active region of a semiconductor laser. By changing the injection current or temperature, the effective refractive index of the grating is modified using the plasma effect and thermal effect, thereby altering its Bragg wavelength to achieve wavelength tuning. It features a narrow tuning range (several nanometers to more than ten nanometers) but fast tuning speed (nanosecond to microsecond level), narrow linewidth, and small size, making it highly suitable for high-speed optical fiber communication systems.

3.2 External Cavity Diode Laser 
One end face of the laser diode (LD) is coated with an anti-reflection film and placed in an external resonant cavity, where a grating serves as the wavelength-selective element. By precisely rotating the angle of the grating, the strongly reflected wavelength fed back into the laser diode can be adjusted, thereby achieving single-mode and wide-range wavelength tuning. It boasts an extremely wide tuning range (up to hundreds of nanometers), an ultra-narrow linewidth, and excellent frequency stability, making it a primary tool in atomic physics, spectroscopy, and high-resolution measurement.

3.3 Tunable Vertical-Cavity Surface-Emitting Lasers 
The resonant cavity of a VCSEL is oriented perpendicular to the substrate surface. By integrating microelectromechanical systems, the upper mirror of the VCSEL can be fabricated as a movable cantilever or thin film. By applying an external voltage, the cavity length can be precisely controlled, enabling continuous wavelength tuning. This technology boasts fast tuning speed, low power consumption, and ease of two-dimensional integration, making it highly promising for applications in high-speed sensing and communication fields.

3.4 Titanium-Sapphire Lasers and Other DPSS Lasers

The titanium-sapphire crystal has an extremely broad gain bandwidth (~650-1100 nm). By using tunable intracavity components (such as birefringent filters), tuning can be achieved over its entire gain range. With an extremely wide tuning range and high output power, it is an important tool for ultrafast laser and broad-spectrum spectroscopy research.

3.5 Fiber Laser 
Fiber doped with rare earth elements (such as erbium, ytterbium, thulium) is used as the gain medium, combined with tunable filters (e.g., tunable Fabry-Pérot filters, acousto-optic tunable filters) as wavelength-selective components. It features high output power and excellent beam quality, with the tuning range depending on the gain fiber (e.g., erbium-doped fiber operates in the C-band and L-band). It is widely used in optical fiber sensing and spectroscopy.

4. Key Parameters

Tunable wavelength lasers have the following key parameters:

4.1 Tuning Range: Whether the output power of the laser is stable over the entire tuning range. The maximum power and power stability are provided.

4.2 Output Power: Whether the output power of the laser is stable over the entire tuning range. The maximum power and power stability are provided.

4.3 Line Width: The width of the laser spectrum determines the laser’s monochromaticity. The narrow linewidth is crucial for high-resolution spectroscopy and coherent communication.

TUN 1054–1074 nm wavelength and linewidth stability (<10 pm over 1 hour ±2℃)

4.4 Tuning Speed: The time required to switch from one wavelength to another. This is crucial for high-speed communication and dynamic sensing.

4.5 Wavelength Accuracy and Repeatability: The accuracy of wavelength setting and the consistency of multiple tunings to the same wavelength.

TUN 1054–1074 nm wavelength fine-tuning range of 23 GHz (89 pm), with tuning accuracy better than 1 pm.

5. Applications of Wavelength-Tunable Lasers 
5.1 Optical Fiber Communication: As a light source for wavelength division multiplexing (WDM) systems, it can flexibly allocate wavelengths, greatly improving network capacity and flexibility.

5.2 Spectroscopy: By scanning the laser wavelength, the absorption and emission spectra of atoms and molecules can be detected with high resolution and sensitivity, which is used in environmental monitoring, chemical analysis, and medical diagnosis.

5.3 Sensing Technology: Sensors based on fiber Bragg gratings, Fabry-Pérot interferometers, etc., measure physical quantities such as stress, temperature, and pressure by detecting wavelength shifts.

5.4 Biomedical Engineering: Optical coherence tomography (OCT) uses a broadband light source for high-resolution imaging of biological tissues, and tunable lasers are its core component.

5.5 Metrology and Standards: Serving as a frequency standard, it is used in precision measurement, atomic clocks, and basic physics research.

5.6 National Defense and Security: Applied in free-space optical communication, lidar, and chemical warfare agent detection.

6. Typical Wavelength-Tunable Laser 
The wavelength-tunable lasers mainly include types such as semiconductor lasers, solid-state lasers, titanium-sapphire lasers, and fiber lasers. The typical wavelength range covers from 400nm to the mid-infrared. With output power above the watt level, the tuning range can reach over 100nm, and the tuning accuracy is less than 0.001nm. These lasers are widely used in fields such as biomedicine, spectral analysis, sensing, and metrology. For more details, please visit www.cnilaser.com/ to learn more.

Tunable Diode Laser	Tunable DPSS Laser	Tunable Titanium-Sapphire Laser	Tunable Fiber Laser

Wavelength-tunable lasers have evolved from precision laboratory instruments into a key enabling technology driving the development of modern information technology, scientific research, and industrial manufacturing. With the advancement of new materials (such as quantum dots and quantum wells), new structures (such as photonic integrated circuits), and intelligent control algorithms, future wavelength-tunable lasers will move toward broader tuning ranges, higher output power, smaller sizes, lower costs, and greater intelligence. They will continue to play an indispensable role in illuminating the unknown world and connecting the digital future.