High Power Ultrafast Laser Mirrors-You Need to Know
Introduction: What are Ultrafast Lasers
Ultrafast Lasers are lasers with pulse widths on the order of femtoseconds (10^-15 seconds) or picoseconds (10^-12 seconds). The extraordinarily short pulse width gives ultrafast pulsed lasers unique properties and a wide range of applications like eye surgeries, laser processing (drilling, cutting), and micro-welding. The ultrashort pulsed lasers unlock new possibilities and potentials as immense peak power becomes accessible at the same average power as continuous wave lasers. Ultrafast lasers have gained unique advantages, such as high spatial precision, less tendency to heat up, and the occurrence of nonlinear processes etc.
Unlike continuous wave (CW) lasers, where the laser energies are released continuously with a constant power level, he fundamental principle of ultrafast pulsed lasers is that the energies are stored and released within very short time durations using the technique of mode-locking. The power levels of ultrafast lasers are uneven, spanning from high peak power values at the pulse peak to zero at the pulse intervals.
In this article, which is we will be looking at different aspects of ultrafast lasers, including how ultrafast lasers are produced (via mode locking and chirped pulse amplification), the optics needed for a ultrafast femtoline laser setup, the core elements for ultrafast laser mirrors the technical challenges for ultrafast laser mirrors.
How are Ultrafast Lasers Obtained- Mode Locking and Chirped Pulse Amplification
When the pulse duration is down to picosecond or femtosecond scale, the ultrafast laser pulses are generated using the method of Mode Locking.
The theoretical basis of the ultrafast laser mode locking technique is to introduce a fixed phase relationship between different modes in the laser resonator. The laser generated in this manner is called a phase-locked laser or mode-locked laser. The interference between these modes will cause the laser to produce a series of pulses. Depending on the nature of the laser, these pulses can have an extremely short duration, even up to the order of femtoseconds.
This is because for multi-longitudinal mode lasers, light of multiple frequencies is emitted, and the macroscopic appearance of the emitted laser in the time domain is closely related to the phase relationship of each longitudinal mode in the resonant cavity. In the state of random phase relationship (that is, light of different frequencies has different phase relationships at different times), the laser outputs continuous lasers, and the light intensity appears chaotic in the time domain; while in the state of specific longitudinal modes and phase relationships, the laser outputs periodic pulses of light (i.e. pulsed laser). The real meaning of the mode-locked state is that all longitudinal modes have a fixed phase relationship that does not change with time.
There are 2 types of mode-locking: passive mode locking and active mode locking.
Passive Mode Locking is generally preferred for generating ultrashort pulses, such as femtoline lasers. Passive mode-locking is achieved by placing a saturable absorber inside the laser resonant cavity. A saturable absorber is a nonlinear absorbing medium that has an absorptive jump to the laser frequency and a large absorption cross-section. Once the laser radiation pulse is incident to this absorber, the absorber molecules absorb the laser radiation, and as the laser intensity increases, the number of particles in its upper energy level also increases, and when the laser intensity is greater than the saturation intensity of the absorber, the absorber reaches saturation, so that the maximum intensity of the laser pulse is subjected to the smallest possible loss and free to pass through it, resulting in a strong mode-locked pulse. It is similar to passive Q-switching, but with differences. Passive mode-locking requires that the upper energy level of the saturable absorber has a particularly short lifetime, and also that it must be placed in the cavity close to the total mirror.
Active Mode Locking uses a method of periodically modulating the parameters of the resonant cavity. The basis is to use a modulator controlled by an external signal in the resonant cavity to periodically change the loss or optical path (amplitude modulation and phase modulation) of the resonant cavity with a certain modulation frequency. When the selected modulation frequency is equal to the interval between the longitudinal modes, the modulation of each mode will produce sidebands, whose frequencies are consistent with the frequencies of the two adjacent longitudinal modes. Due to the interaction between the modes, all modes can be synchronized under sufficiently strong modulation, and the modes will coherently superimpose to form a mode-locked sequence pulse.
For ultra-short pulse durations, the peak power can become massive, for example, from Gigawatts (GW), terawatts (TW), or even Petawatts (PW). While the high peak powers have unraveled benefits, problems also arise. The Chirped Pulse Amplification (CPA) technique is a very useful technique for preventing the enormous peak power from bringing damage to the laser amplifier or introducing unwanted nonlinear pulse distortion.
The process of CPA can be divided into:
1. Pulse stretching: The short pulse is sent through dispersive optics to temporarily stretch it in time. And since the pulse duration is increased, the peak power is lowered.
2. Amplification: The stretched, lower-peak-power pulse is then amplified using conventional laser amplifiers.
3. Pulse compression: The laser pulse is recompressed to its original ultrafast pulse duration using dispersion compensation optics, such as chirped mirrors.
Optics Needed for Ultrafast Lasers
Below is the basic setup structure of an ultrafast femtosecond laser:
1. Seed Laser / Oscillator
Function: Generates the initial train of femtosecond pulses (typically 10 fs to 300 fs).
Common type: Ti: Sapphire mode-locked laser (operating around 800 nm).
Repetition rate: Often in the range of tens to hundreds of MHz.
Output power: Usually a few hundred milliwatts.
2. Pulse Picker (Optional)
Function: Reduces the pulse repetition rate (e.g., from 80 MHz down to kHz range) for high-energy amplification.
Type: Acousto-optic modulator (AOM) or electro-optic modulator (EOM).
3. Chirped Pulse Amplification (CPA)
Function: For amplification of ultrashort pulses without damaging the gain medium or causing nonlinear distortion
a. Pulse Stretcher
Function: Temporarily stretches femtosecond pulses to picoseconds or nanoseconds to reduce peak power.
Method: Grating or prism-based dispersive optics.
b. Amplifier
Function: Amplifies the stretched pulses.
Types:
Regenerative amplifier (Ti:Sapphire, Yb-doped, or fiber-based).
Multi-pass amplifier.
Output power: From milliwatts to several watts, depending on application.
c. Pulse Compressor
Function: Recompresses the amplified pulse back to femtoseconds.
Method: Uses gratings, prisms, or chirped mirrors.
4. Optional Modules (Depending on Application):
Harmonic generation crystals (for frequency doubling/tripling).
Optical parametric amplifier (OPA) for tunable wavelengths.
5. Beam Delivery Optics
Includes: high power laser mirrors, focusing lenses, beam expanders, isolators, optical filters
Purpose: Steer, converge, or collimate the laser beam, selecting the output wavelength
The Heart of Ultrafast Lasers-Ultrafast Laser Mirrors
At the heart of ultrafast lasers lies ultrafast laser mirrors, which are the essential components of the laser oscillator cavity of ultrafast lasers. The mirror of an ultrafast laser must have a high laser-induced damage threshold (LIDT) to endure high peak power, low GDD to maintain the pulse shape (when the pulse duration is down to femtoseconds), and high reflectivities.For ultrafast lasers with pulse durations shorter than nanoseconds, the laser induced damaging mechanism is not thermal damage, as in the case of CW lasers, but nonlinear processes like multiphoton absorption and ionization. Direct conversion of the LIDT at ns to ps or fs doesn’t work; the best practice is to have your optics tested with exactly the operation wavelength, pulse duration, and repetition rate.
In optical systems, group delay dispersion (GDD) is a physical quantity, often measured in fs^2, that describes the difference in transmission speeds of different components of frequencies during the propagation of an optical pulse. When an ultrashort pulse is transmitted in a medium, its spectral components experience different propagation times, and the second-order derivative of this time delay with frequency is defined as the group delay dispersion parameter.
In the field of ultrafast optics, the GDD directly affects the pulse width and shape. When positive GDD exists, the high-frequency component propagates more slowly than the low-frequency component, resulting in a pulse with a higher frequency at the trailing edge than at the leading edge, and this dispersion effect broadens the pulse time domain. Large GDD elongates the pulse and results in pulse distortions, which is why it is undesirable. However, the effects of GDD also depend on the pulse length, usually, GDD is only a matter of concern at femtosecond pulsed durations; for longer pulses, it doesn’t matter.
Ultrafast laser mirrors are designed to have low GDD Two common mirror types include ultrafast enhanced silver mirrors or low GDD dielectric mirrors, which are all available in Shalom EO. In Shalom EO, we utilize Ultrafast Innovation GOBI white light interferometer to precisely control the GDD of ultrafast laser mirrors.
Some special dispersive optics for ultrafast lasers are engineered with a specific positive GDD value for pulse stretching or a negative GDD value for pulse compression, typical examples are grating or prism pairs, chirped mirrors, or chirped mirror pairs. The pulse stretching optics are critical elements for chirped pulse amplification, while the pulse-compressing optics, like chirped mirrors, are needed when a fine temporal resolution is required for your application or to restore the initial pulse length in chirped pulse amplification.
Shalom EO: When Reflectance Meets Resilience
Hangzhou Shalom EO offers both standard and custom ultrafast laser mirrors for femtosecond lasers. Our mirrors feature advanced dielectric coatings applied to UV-grade fused silica substrates, which have the lowest GDD value in the visible or NIR range. The UV fused silica is sourced from Corning (Corning 7980). The laser-induced damage threshold (LIDT) of our 950-1100nm laser mirror is 2J/cm2@1030nm,100ps,100hz, and for the 750-850nm laser mirror is 300mJ @800nm,100fs,100hz—ideal for high-energy pulsed lasers. What’s more, our high-power ultrafast laser mirrors maintain superior reflectivities of >99.9% at critical wavelengths, with coatings optimized for specific AOIs and polarization states.We also recommend our chirped mirrors and chirped mirror pairs, designed with reflectance of >99.9% for AOIs of 0-10°, available in standard or custom diameters from 0.5” to 10”. The chirped mirrors are laser mirrors with a special dielectric coating thickness structure, where different wavelengths penetrate different depths of the coatings, producing negative group delay dispersion (GDD). Compared with optical gratings or prisms, the advantages of chirped mirrors include that they are easier to align and broader wavelength range. Chirped mirrors are ideal for chirped-pulse amplifiers and ultra-broadband laser oscillators. The complementary chirped mirror pairs are optimized to even out the GDD fluctuations.
In Shalom EO, we have expert coating capabilities, the coatings are computer-engineered and designed, and then fabricated in our own class 1000 clean room using our SHINCRON MIC-1350TBN Ion Assisted Electron Beam Deposition (IAD e-beam) coating machine. Click here to learn more about Shalom EO’s optical coating capabilities for ultrafast and high-power coatings.
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