Explore 755nm Alexandrite Laser Optics
Alexandrite lasers are laser sources that adopt the alexandrite crystal as the laser gain medium, operating at a wavelength of 755nm, and are among the most versatile and effective laser systems available today. Their unique properties make them essential tools in both medical and aesthetic applications.
In this article, we will explore the uses of Alexandrite lasers, compare them with diode lasers, and shed light on the optical components that ensure their optimal performance.
What are Alexandrite Lasers Used For?
Alexandrite lasers use the gemstone Alexandrite crystals as the laser generation medium. Alexandrite crystal, with the chemical formula Cr3+:BeAl2O4 is a highly anisotropic crystal material, although there are naturally occurring Alexandrite crystals, ones used as laser crystals are synthetically grown. The most typical laser emission wavelength of Alexandrite crystal is around 755nm, where the laser achieves its highest gain, meaning the light amplification gets most efficient. The basic working principle to make laser gain media emit laser is to use certain external sources of energy to excite the media, where the excited atoms undergo stimulated emission inside optical cavities, producing coherent photons that are amplified into laser beams. The most common approaches to pump alexandrite include lamp pumping and end pumping.
In lamp pumping of Alexandrite lasers, a powerful flashlamp (like a xenon or krypton arc lamp) surrounds or sits near the Alexandrite crystal. The lamp emits broad-spectrum intense light (covering visible and infrared wavelengths), and the Alexandrite crystal absorbs part of this light to become excited and ready to emit lasers.
In the end pumping of Alexandrite lasers, a laser diode (or sometimes another focused light source) shines directly into one end of the Alexandrite crystal. The pump light is narrower in wavelength (more spectrally matched to Alexandrite’s absorption bands), so the crystal absorbs the energy much more efficiently.
Alexandrite lasers are renowned for their excellent tissue penetration and selective photothermolysis, making them highly effective for a range of dermatological and aesthetic procedures like hair removal, tattoo removal, lesion removal, etc..
The 755nm Alexandrite lasers used for skin care and cosmetic purposes are usually pulsed lasers with pulse durations shorter than nanosecond scale (1ns=10^-9s), down to picosecond scale (1ps=10^-12s). This means these alexandrite lasers don't emit a continuous beam of light. Instead, they deliver very short, intense bursts of laser energy. The reason why short pulsed lasers are used instead of continuous wave lasers is that short pulses allow the laser to rapidly deliver energy to the target (like melanin in hair follicles or tattoo ink particles) before the heat can spread to the surrounding skin, thus improving precision and reducing the risk of skin damage. Moreover, at picosecond durations, the laser creates a strong mechanical shockwave effective for breaking apart pigment particles (for tattoo removal) or treating certain types of pigmented lesions. In addition, shorter pulse durations lead to higher peak power without increasing the total energy. In conclusion, short pulsed Alexandrite crystal lasers improve precision, safety, and treatment effectiveness in aesthetic treatment applications.
1. Hair Removal:
The 755 nm wavelength is strongly absorbed by melanin (the pigment found in hair shafts and hair follicles), making Alexandrite lasers ideal for removing dark hair, particularly on lighter skin types (Fitzpatrick I–III). When the melanin absorbs the laser energies, thermolysis takes place, the heat damages or destroys the hair follicle cells responsible for hair growth, especially the stem cells in the bulge region and cells in the hair bulb. 755nm alexandrite lasers are advantageous compared to other longer wavelengths (e.g., Nd: YAG lasers) for removal of finer hairs, however, caution must be taken for patients with darker skin tones or tanned skin, because the alexandrite laser damages melanin, which mainly exists in hair but also exists in dark skin.
2. Pigmented Lesions:
Alexandrite lasers are commonly used to treat pigmented lesions such as age spots, freckles, and café-au-lait spots. The high absorption by melanin allows precise targeting of pigmented cells with minimal damage to surrounding tissue.
3. Vascular Lesions:
Although less commonly used for vascular treatments compared to longer wavelengths, Alexandrite lasers can still treat certain vascular lesions with effectiveness, such as small facial veins, leg veins, where the Light pulses target red pigment (haemoglobin).
4. Tattoo Removal:
To remove a tattoo, you must help the human body break down the pigment into pieces small enough that macrophages can successfully remove them. The 755 nm output is particularly effective for removing blue, purple, green, and black tattoo inks. Its selective absorption properties allow for the targeted breakdown of ink particles with fewer treatment sessions.
Diode Lasers Vs Alexandrite Lasers
While both diode and Alexandrite lasers are popular choices for aesthetic and dermatological treatments, they differ significantly in their characteristics:
Feature | Alexandrite Laser (755 nm) | Diode Laser (typically 800–810 nm) |
Wavelength | 755 nm | 800–810 nm |
Melanin Absorption | Higher | Moderate |
Skin Types | Best for I–III | Suitable for I–V |
Penetration Depth | Moderate | Deeper |
Treatment Speed | Faster | Moderate |
Pain Level | Slightly higher | Typically lower with cooling |
Applications | Hair removal, pigmentation, tattoo removal | Hair removal, vascular treatments |
In general, Alexandrite lasers offer faster results for hair removal and pigmentation treatments, particularly for patients with lighter skin tones. Diode lasers, on the other hand, are more versatile across a wider range of skin types and often come integrated with advanced cooling systems to enhance patient comfort.
755nm Alexandrite Laser Optics
The performance of an Alexandrite laser system heavily depends on the quality of its optical components. Specialized optics are required to handle the unique demands of 755 nm operation, including high energy densities and strict beam quality requirements.
Critical optical components include:
1. Laser Rods:
The Alexandrite crystal itself serves as the gain medium. High-purity crystal growth and precise fabrication are critical for achieving stable and efficient laser output.
2. Q-Switch/Pockel cells
To generate pulsed output from a 755 nm Alexandrite laser, an optical device called a Q-switch is commonly used. In a basic continuous-wave (CW) Alexandrite laser, the laser crystal constantly emits light as soon as enough energy is pumped into it. But to create short, high-energy pulses, you need a way to store energy in the gain medium first, then release it all at once. Q-switching is the technique that does this.
Pockel cells are the core element of electro-optic q-switching. When realizing electro-optic q-switching, Pockels cells are often used in combination with polarizers. Here’s the process of Q-switching:
- Low-Q state (blocking laser oscillation)
A high voltage is applied to the Pockels cell, rotating the polarization of the light (typically by 90°).
After passing through the Pockels cell, the rotated light hits a polarizer and gets blocked or diverted.
The laser cavity has high losses, preventing lasing, but the gain medium continues building up population inversion (stored energy).
- High-Q state (releasing the laser pulse)
The voltage is suddenly removed (or switched appropriately), so the Pockels cell stops rotating the polarization.
Light now passes straight through the Pockels cell and polarizers without loss.
The cavity switches to a low-loss state.
With a high population inversion already built up, the laser quickly releases a powerful pulse.
3. Output Couplers and Mirrors:
High-reflectance mirrors and output couplers with coatings optimized for 755 nm are essential. Coatings must withstand high-power densities and provide excellent thermal stability.
4. Lenses:
Focusing and beam-shaping optics, such as plano-convex or aspheric lenses made of high-damage-threshold materials, ensure minimal absorption and high transmission at 755 nm.
5. Beam Delivery Systems:
Optical fibers, articulated arms, or free-space optics guide the laser beam to the treatment area. Anti-reflection coatings and precise alignment are crucial to maintain beam quality and energy delivery.
6. Protective Windows and Filters:
To safeguard internal components and users, protective windows with specialized coatings block unwanted wavelengths while transmitting the 755 nm laser beam efficiently.
Careful selection and maintenance of these Alexandrite laser optics are vital for maximizing laser efficiencies, prolonging system life, and ensuring consistent treatment results.
Shalom EO-Supplier of Alexandrite Laser Optics
Hangzhou Shalom EO is a specialized supplier of optics for Alexandrite lasers, We can provide a complete series of 755nm Alexandrite laser optics in off-the-shelf and custom versions, including the following:
-Alexandrite Laser Crystals
Shalom EO’s Alexandrite laser rods feature a high damage threshold > 1.5 million pulses ( tested by a third party), and high-quality quality excellent for 755nm long-pulsed, Q-Switched, or picosecond lasers. We also provide refurbishing services (re-polishing and re-coating for used laser rods).
-755nm Electro-optical Q-switches (Pockels Cells)
Pockels cells are an optical modulator that controls the polarization state of light via applied voltage. Pockels cells are the core element of electro-optic q-switching. The working principle of Pockel cells is the Pockels effect (i.e., the phenomenon that the refractive index of a medium varies in proportion to the applied electric field strength), which occurs in certain crystal materials like LiNbO3, DKDP, and BBO. Hangzhou Shalom EO provides 755nm wavelength Pockels cells developed based on DKDP crystals. We can offer DKDP Pockels cells with 3-pin structures.
-755nm Laser Line Mirrors
We can provide laser line mirrors optimized for 755nm Alexandrite laser systems.
-Optical coating capabilities
Shalom EO can design and fabricate various optical coatings, including the optical coatings oriented for 755nm Alexandrite laser optics, such as anti-reflection coatings, high-reflective coatings, optical filter coatings, low GDD/high power ultrafast coatings, and more. In Shalom EO’s factory, empowered with engineers of specialized knowledge, and our advanced coating and inspection equipment like SHINCRON MIC-1350TBN IAD e-beam coating machine, PerkinElmer Lambda 1050+ spectrometer, Ultrafast Innovations GOBI white light interferometer, etc., we guarantee the enduring precision and quality of our optical coatings.
-Others:
Shalom EO also has expert experience and technical competence in providing optical filters, lenses, and optical couplers (ball lenses and half ball lenses) for 755nm Alexandrite lasers.
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