The normal mode, also known as free-running pulse mode, functions by allowing the laser pulse duration to be determined naturally by the physical properties of the laser medium rather than electronic gating. Unlike Q-switching, which compresses energy into nanosecond bursts for mechanical impact, normal mode delivers energy over a duration of 100ns to 100µs to create a purely thermal effect necessary for destroying hair follicles.
By foregoing pulse compression, normal mode ensures the energy delivery is sufficiently long to heat the hair follicle through to destruction without causing the explosive mechanical tissue damage associated with shorter, Q-switched pulses.
The Mechanics of Free-Running Mode
Pure Thermal Generation
The primary function of normal mode is to facilitate selective photothermolysis.
In this mode, the laser emits energy as long as the excitation source is active, governed by the laser medium's energy levels. This results in a "long" pulse (relative to Q-switching) that allows heat to accumulate within the target structure, effectively cooking the follicle rather than shattering it.
Contrast with Q-Switching
It is vital to understand what normal mode is not.
Q-switched lasers produce extremely short, high-peak-power pulses designed to shatter pigment (like tattoo ink) acoustically. Normal mode avoids this "photoacoustic" effect. Instead, it provides a steady stream of photons that convert to heat upon absorption, which is the specific mechanism required to permanently damage the hair bulb and bulge.
The Role of Pulse Duration in Safety
Matching Thermal Relaxation Time (TRT)
While normal mode defines the operational state, the resulting pulse duration is critical for targeting.
The duration must be calibrated to the Thermal Relaxation Time of the hair. This is the time it takes for the target to cool down by 50%. The laser pulse must be shorter than the hair's TRT to damage the follicle, but long enough to prevent the heat from diffusing rapidly into the surrounding tissue.
Protecting the Epidermis
The pulse duration in normal mode plays a dual role: destroying the hair while saving the skin.
The epidermis (top layer of skin) has a much shorter cooling time than the hair follicle. By utilizing a specific pulse width—often extended in modern applications—the system allows the melanin in the skin to dissipate heat through thermal conduction, while the larger hair follicle retains the lethal heat.
Controlled Energy Delivery
Normal mode supports the use of calibration and safety systems.
Because the beam is not an instantaneous, explosive burst, the system can more effectively direct the laser into safety beam stops or calibration ports during preheating. This ensures that radiation is strictly contained until the precise moment of therapeutic delivery.
Understanding the Trade-offs
The Risk of Heat Diffusion
The greatest challenge with free-running or longer pulse modes is non-specific thermal damage.
If the pulse duration exceeds the thermal relaxation time of the target significantly, heat will diffuse from the follicle into the surrounding dermis. This can lead to collagen damage, pain, and burns.
Precision vs. Efficacy
Finding the exact balance is difficult because biological targets vary.
A pulse that is too short may fail to heat the entire follicle uniformly, leading to regrowth. Conversely, a pulse that is too long (relative to the specific patient's physiology) risks overheating the surrounding skin, particularly in patients with darker skin tones where epidermal melanin absorbs significant energy.
Making the Right Choice for Your Goal
The "normal mode" establishes the fundamental physics of the laser, but how you utilize pulse duration within that mode dictates clinical success.
- If your primary focus is Coarse Hair: You generally require a longer pulse duration to ensure heat effectively conducts from the shaft to the entire follicle wall without dissipating too quickly.
- If your primary focus is Darker Skin Tones: You must extend the pulse duration to allow the epidermis sufficient time to dissipate heat, preventing surface burns while still treating the follicle.
- If your primary focus is Fine/Light Hair: You require a shorter pulse duration (closer to the lower end of the mode's capability) because smaller targets cool down much faster and need rapid energy delivery to reach the destruction threshold.
Ultimately, normal mode provides the foundational "long pulse" architecture required to convert light into controlled heat, making safe hair reduction physically possible.
Summary Table:
| Feature | Normal/Free-Running Mode | Q-Switched Mode |
|---|---|---|
| Mechanism | Pure Thermal Effect (Heating) | Photoacoustic Effect (Shattering) |
| Pulse Duration | 100ns to 100µs (Longer) | Nanoseconds (Ultra-short) |
| Primary Goal | Hair follicle destruction | Pigment/Tattoo removal |
| Tissue Impact | Controlled heat accumulation | Explosive mechanical impact |
| Key Benefit | Selective photothermolysis | High peak power for fragmentation |
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References
- F W Cross. Laser Hair Removal. DOI: 10.1046/j.1473-2130.2001.00007.x
This article is also based on technical information from Belislaser Knowledge Base .
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