A dichroic mirror facilitates energy extraction by acting as a wavelength-selective filter within the laser resonator. Using specialized thin-film interference coatings, the mirror remains highly reflective for the fundamental 1064nm light to sustain the lasing process, while simultaneously becoming highly transmissive (or reflective at specific angles) for the frequency-doubled 532nm green light, allowing it to exit the system.
By decoupling the exit path of the green light from the trapped fundamental beam, dichroic mirrors ensure efficient output and prevent the high-energy green light from re-entering the gain medium where it causes signal loss.
The Mechanics of Wavelength Separation
Thin-Film Interference Coatings
The core function of a dichroic mirror relies on precise thin-film interference coatings.
These are not standard metallic reflections; they are engineered layers that interact differently with specific wavelengths.
For a 532nm laser system, the coating is tuned to manage two distinct behaviors on a single surface.
Trapping the Fundamental 1064nm
To maintain the laser's operation, the fundamental infrared light (1064nm) must remain inside the resonator.
The dichroic mirror provides High Reflectivity (HR) for this specific wavelength.
This keeps the photon density high within the cavity, ensuring the stimulated emission process continues efficiently.
Releasing the Green 532nm
Once the fundamental light passes through a non-linear crystal and converts to green light (532nm), it needs to be extracted immediately.
The dichroic mirror is designed to be Highly Transmissive (HT) or reflective at a specific angle for this new wavelength.
This allows the green light to "break out" of the feedback loop that traps the infrared light.
Optimizing Output with Cavity Geometry
The U-Shaped Cavity Strategy
The reference highlights the effectiveness of placing these mirrors within a U-shaped cavity.
In this configuration, the dichroic mirror acts as a folding mirror or an output coupler.
It directs the fundamental beam back into the gain medium while allowing the generated green beam to exit linearly.
Preventing Unnecessary Loss
A critical role of the dichroic mirror is protecting the system's efficiency.
If 532nm light is allowed to reflect back into the gain medium, it is often absorbed or scattered, resulting in thermal lensing or output instability.
By strategically exporting the green light away from the gain medium, the mirror minimizes these losses.
Understanding the Trade-offs
Angular Sensitivity
Dichroic coatings are highly sensitive to the angle of incidence.
The reference notes that reflectivity and transmission occur at "specific angles."
If the mirror is slightly misaligned, the spectral properties shift, potentially trapping the green light or leaking the infrared light.
Coating Complexity
Achieving high reflectivity for one wavelength and high transmission for its harmonic (half the wavelength) requires complex manufacturing.
These coatings must be extremely precise.
Imperfections in the coating can lead to scattering, which degrades the overall quality of the laser beam.
Designing for Maximum Efficiency
When integrating dichroic mirrors into a resonator, your design choices depend on your specific stability and power requirements.
- If your primary focus is Output Power: Ensure the mirror is placed immediately after the frequency-doubling crystal to extract green light before it incurs propagation losses.
- If your primary focus is Beam Quality: Utilize a U-shaped cavity geometry to strictly separate the thermal load of the gain medium from the generated green output.
Success in green laser design ultimately hinges on the precision of the dichroic coating's ability to discriminate between the fundamental and the doubled frequency.
Summary Table:
| Feature | Fundamental Beam (1064nm) | Second Harmonic (532nm) |
|---|---|---|
| Mirror Property | High Reflectivity (HR) | High Transmissivity (HT) |
| Function | Traps light in the resonator | Extracts energy as output |
| Role in Cavity | Maintains stimulated emission | Prevents thermal loss in gain medium |
| Key Constraint | Must remain inside the loop | Must exit the system immediately |
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References
- Tingting Lu, Weibiao Chen Weibiao Chen. Highly efficient single longitudinal mode-pulsed green laser. DOI: 10.3788/col201311.051402
This article is also based on technical information from Belislaser Knowledge Base .
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