The femtosecond laser (1 fs = 10⁻¹⁵ seconds) generates femtosecond-scale ultrashort pulses. As a core ultrafast laser, its ultra-short interaction time and ultra-high peak power open new avenues for basic research and high-precision industrial processing. This article systematically covers its principles, classifications, applications, key parameters, and typical products.

1.Working Principle

The core principle of femtosecond lasers is mode-locking, and it is first necessary to understand how they generate such ultra-short pulses. A laser resonator contains numerous longitudinal modes with different frequencies and phases simultaneously. Mode-locking technology uses a method (active or passive) to maintain a fixed phase relationship between these longitudinal modes of different frequencies (i.e., "locking" the phases). When these in-phase longitudinal modes propagate in the resonator, they coherently superimpose at specific moments to form an intense spike (i.e., a pulse); at other moments, they cancel each other out. Through precise control, thousands of longitudinal modes oscillate synchronously, ultimately outputting a series of ultra-short pulses with fixed intervals and extremely narrow widths in the time domain. Femtosecond lasers utilize this principle to highly concentrate light energy in time, thereby generating extremely high instantaneous power.

The Structure and Operating Principle of a Mode-Locked Laser

2. Classification of Femtosecond Lasers 
Femtosecond lasers can be classified according to their gain medium and mode-locking techniques.

2.1 Classification by Gain Medium: 
(1) Titanium Sapphire Lasers: A classic and versatile femtosecond laser. The titanium sapphire crystal features an extremely wide gain bandwidth, enabling the generation of ultra-short pulses (down to a few femtoseconds) with a typical wavelength tuning range of 680 nm to 1100 nm. It is the preferred choice for many scientific research laboratories.
(2) DPSS Femtosecond Lasers: Utilize ytterbium-doped crystals such as Yb:KYW and Yb:KGW as gain media. They excel in delivering high average power and high pulse energy, serving as key light sources for industrial micromachining.
(3) Fiber Femtosecond Lasers: Employ rare-earth-doped fibers (e.g., erbium (Er), ytterbium (Yb)) as gain media. Their advantages include compact structure, high stability, maintenance-free operation, and excellent heat dissipation, making them ideal for industrial environments. Pulse widths typically range from tens to hundreds of femtoseconds.

2.2 Classification by Clamping Technology: 
(1) Kerr Lens Mode-Locking (KLM): Utilizes the Kerr effect (refractive index varies with light intensity) of the gain medium itself (e.g., titanium sapphire) to form an equivalent "fast saturable absorber," enabling self-starting passive mode-locking. It is the most commonly used method for generating femtosecond pulses in titanium sapphire lasers.
(2) Semiconductor Saturable Absorber Mirrors (SESAMs): An integrated passive mode-locking device that initiates and maintains the mode-locked state through the saturable absorption characteristics of semiconductor materials. It makes laser design more compact and stable, widely used in fiber and solid-state femtosecond lasers.
(3) Nonlinear Amplifying Loop Mirrors (NALMs): A mode-locking technology commonly used in fiber lasers, which achieves pulse formation by introducing nonlinear phase shifts and interference in the ring cavity.

3. Key Parameters of Femtosecond Lasers 
Key parameters of the femtosecond laser include the following:

3.1 Pulse Width: 
The pulse duration is a core parameter of femtosecond lasers. It directly determines the resolution of the processing and the degree of nonlinearity in the interaction with materials. The shorter the pulse, the higher the peak power and the smaller the heat-affected zone.

Waveform of a typical CNI femtosecond laser 
 

3.2 Repetition Frequency
The number of pulses output per second, measured in Hertz (Hz), ranges from MHz (e.g., 80 MHz) to GHz. High repetition rates are ideal for high-speed machining and frequency comb applications, while lower repetition rates—often achieved with the help of an amplifier—can deliver significantly higher single-pulse energy.

3.3 Single-Pulse Energy and Average Power
Average power = Single-pulse energy × Repetition frequency. 
Single-pulse energy determines how much "work" a single pulse can accomplish, such as whether it can induce a specific nonlinear process.

3.4 Peak Power
The ratio of single-pulse energy to pulse width. Due to the extremely short pulse width, femtosecond lasers can achieve astonishing peak powers (for example, a pulse with 1 microjoule of energy and a 100-femtosecond duration can deliver a peak power as high as 10 GW).

3.5 Center Wavelength and Spectral Bandwidth
According to the Fourier transform limit, pulse width is inversely proportional to spectral bandwidth. To achieve shorter pulses, a broader spectrum is required. The wide-spectrum characteristics of titanium-sapphire lasers enable them to generate the shortest possible pulses.

3.6 Pulse Quality
These parameters—including time contrast (whether unwanted pre-pulses or pedestals exist before and after the main pulse) and beam quality (represented by the M² factor)—directly influence the final outcome of the application.

CNI Typical Femtosecond Laser M ² Test Chart

4. Typical Applications of Femtosecond Lasers 
The "cold processing" characteristic (minimal heat-affected zone, HAZ) and ultra-high peak power of femtosecond lasers have enabled them to shine brightly in numerous fields.

4.1 Scientific Research 
Ultrafast Spectroscopy:
Like a "high-speed camera" with a femtosecond shutter, it is used to observe ultrafast dynamic processes at the molecular and atomic scales, such as bond breaking and formation in chemical reactions, and energy transfer in photosynthesis.

High-Harmonic Generation (HHG) & Attosecond Pulses:
Femtosecond lasers interact with matter to generate high-frequency harmonics, which are then used to synthesize attosecond pulses (1 as = 10⁻¹⁸ seconds) for studying electron dynamics.

Optical Frequency Combs:
The frequency domain of a femtosecond laser consists of a series of discrete spectral lines with uniform spacing, like an "optical ruler." It is used for precision distance measurement, optical atomic clocks, and spectroscopic measurements, and was awarded the 2005 Nobel Prize in Physics.

4.2 Precision Micro- and Nanofabrication 
Processing of Transparent Materials:
Leveraging the multiphoton absorption effect, 3D selective engraving, drilling, and optical waveguide fabrication can be performed inside transparent materials. It is used for manufacturing microfluidic chips, optical data storage, etc.

Cutting of Brittle Materials:
Precisely cuts sapphire, glass, ceramics, and other brittle materials with almost no cracks or molten residues.

Surface Microstructures:
Fabricates micro-nano structures with special functions (e.g., anti-reflective, hydrophobic/hydrophilic) on the surfaces of metals, polymers, and other materials.

FS-F-1030 20W Micro-Nano Machining 

4.3 Semiconductor Testing and the Electronics Industry 
Femtosecond lasers can be used for cutting PCB/FPC flexible circuits and even applied in the lithography process of integrated circuits (ICs).

Wafer and PCB/FPC flexible circuit cutting 
 

4.4 Ophthalmic Surgery (LASIK) 
Femtosecond lasers have replaced mechanical microkeratomes for creating precise corneal flaps in LASIK surgery. Their high precision and excellent predictability significantly enhance surgical safety and outcomes.

Femtosecond Ophthalmic Surgery 
 

4.5 Medical and Biomedical Imaging 
Femtosecond lasers are the ideal light source for two-photon/three-photon microscopy imaging, enabling deep, high-resolution, and non-invasive three-dimensional imaging of living tissues—making them a crucial tool in neuroscience and cancer research.

Two-photon microscopy

5. Typical femtosecond laser 
CNI's femtosecond lasers, developed and manufactured in-house, feature typical wavelengths such as 266nm, 343nm, 355nm, 515nm, 532nm, 780nm, 1030nm, 1064nm, and 1560nm, delivering exceptional high beam quality and stability. With their compact packaging and either air-cooled or water-cooled thermal management systems, these lasers are an excellent choice for applications in industrial processing, scientific research, biomedical instrumentation, and more. 
For details, please visit www.cnilaser.com/ Learn more.

Optical fiber femtosecond laser	Solid-state femtosecond laser	Titanium-sapphire femtosecond laser

Femtosecond lasers, with their unique temporal domain characteristics, have become a key engine driving cutting-edge scientific research and the development of high-tech industries. Their influence is ubiquitous—from revealing the fastest microscopic processes in nature, to manufacturing precision components in next-generation consumer electronics, and even revolutionizing medical surgical methods. In the future, as technology advances, femtosecond lasers are moving toward higher power, shorter pulses, lower costs, and smaller sizes, and will surely open up more exciting new application areas.