We may say that laser diodes are all single wavelength in the traditional sense. I believe that if you go deep into the laser field earlier, you should know about dual-wavelength lasers. Then do you know how they are made? Let's take a look at it together.

Dual-wavelength LD = two laser chips+one optical path synthesis structure

It doesn't grow 650nm and 780nm materials in one chip at the same time.

At present, there are three mainstream manufacturing schemes in the industry: double chip stacking, built-in optical beam combining and single package and double lasers.

These three methods all revolve around a core idea: two independent chips, the wavelengths do not interfere with each other, and the two beams of light are combined into one output through optics.

The laser wavelength is essentially determined by the material, and it is impossible to rely on "one LD emitting two kinds of light at the same time". Look at 650nm and 780nm, their materials are completely different: 650nm red light: AlGaInP series, 780nm near infrared: GaAs series.

These two epitaxial systems can't grow together at all

(Unless the multi-quantum well hybrid structure is made, but the cost is extremely high, the industry will not do this).

Therefore, the industry adopts the most pragmatic way: separating the physical structure and combining photosynthesis.

Key technical difficulties of dual-wavelength LD

Optical axis alignment

The positions of luminescent spots at 650nm and 780nm are different.

When combining beams, it is necessary to achieve: vertical deviation < 20 µm, horizontal deviation < 30 µm, and high spot overlap.

Spectral selectivity of diaphragm

Dichroic filter must be highly reflective to 650nm, highly transmissive to 780nm, or vice versa, and the angle should be within 1, the temperature drift should not be too large, and the surface roughness should be low. This is not something that can be done by simply plating a film.

Heat dissipation and driving

When the two LD's work at the same time, the calorific value is almost doubled, the wavelength drifts due TO the temperature rise, an independent driving circuit is needed, and the TO pin must be customized additionally.

These are how to make 650nm and 780nm dual-wavelength laser diodes. I hope you will have a clear understanding after reading them.