Diode Lasers

 

• Emit wavelengths in the visible (red) and near IR
• Output power up to 25 mW
• Integral control electronics
• Integral protective circuitry
• Modulatable versions
• Integral beam-collimating optics
• Mate to LINOS systems Microbench and Nanobench
• Compact dimensions
• Lightweight
• OEM version (DS 670 OEM and x.ldm serie)
• Long lifetime

 

 

How diode lasers work

 

Diode lasers, also called "laser diodes" or semiconductor lasers, are configured from doped semiconductor structures.

 

Superimposing layers of p-type and n-type semiconductors creates ultrathin p-n junctions, with shifts in energy bands occurring across these junctions.

 

When these types of diodes are connected to voltage sources, observing the correct polarity for forward bias, i.e., with the positive terminals of voltage sources connected to their p-type contacts and the negative terminals of voltage sources connected to their n-type contacts, then holes from their p-type layers, and electrons from their n-type layers will be injected into these junctions, thereby creating, by highly efficient mechanisms, population inversions within these junctions. Electrons and holes can then recombine, spontaneously emitting photons in the process.

 

If these spontaneously emitted photons induce emission of further photons having the same energies (frequencies), traveling in the same directions, and having the same polarizations, then stimulated emission occurs.

 

As for all other types of lasers, laser emission will only occur if the number of photons emitted due to stimulated emission exceeds losses due to absorption, and to photons exiting the active region. The diode currents at which laser emission is initiated are termed their "threshold" currents.

 

The unpolished facets of semiconductor crystals usually act as crude "mirrors" reflecting roughly 30% of incident radiation at optical wavelengths. The wavelengths emitted by diode lasers will largely depend upon the properties of the semiconductor materials employed in fabricating their structures, and will typically vary from wavelengths falling within the infrared spectral region down to visible wavelengths.

 

 

Since they have much higher efficiencies than the homostructure diode lasers described above, so-called "double-heterojunction" diode lasers are currently the preferred choice for CW-operation. These latter types of diode lasers have two multiply-doped semiconductor layers applied to their top and bottom faces, oriented orthogonally to long axes of their active regions, that form their resonators through the abrupt transitions in refractive index occurring at these additional interfaces. So-called "index-guided" diode lasers, crystalline structures in which the lateral facets of active regions are also bounded by abrupt shifts in refractice index, have been developed in order to further confine active regions. This approach results in lower threshold currents and significantly reduces beam astimagtism. It also narrows the widths of their emission spectra to the point where these types of diode lasers emit only a single longitudinal mode.

 

 

 

Beam divergence/beam profile

 

 

The beam-exit surfaces of diode lasers have extremely small areas whose spatial extensions transverse to their p-n junctions are frequently of the same order of magnitude as their lasing wavelengths. Diffraction effects at their beam exits thus lead to their beams being highly divergent. The dimensions of their active regions parallel to their p-n junctions exceed those in the orthogonal direction, which explains why diode-laser beams have different divergence angles, Q_|| and Q_^, along these two axes, producing their roughly elliptical beam profiles.

Beam astigmatism

 

 

Beams from diode lasers usually exhibit astigmatism, i.e., their beams appear to originate from two different points, depending upon whether they are viewed parallel or orthogonal to the planes of their p-n junctions. The degree of this astigmatism is determined by the spatial separation, DZ, of these two points. This effect is relatively minor in the case of index-guided diode lasers, but will still require correction if their beams are to be focused down to very small spots.

 

 

 

Beam polarization

 

Above their lasing thresholds, diode laser beams are plane-polarized parallel to the planes of their p-n junctions. Since spontaneous emission is unavoidable, their beams will invariably contain some unpolarized light. However, their beam polarizations improve at higher output powers.

 

Beam spectral characteristics

 

Multimode diode lasers emit several longitudinal modes differing both in intensity and wavelength. Their relatively large longitudinal mode spacings of about 0.2 nm are due to their extremely short resonators.

 

Single-mode diode lasers emit only one longitudinal mode, the reason for their large coherence lengths.

 

Thermal drift

 

 

Wavelengths emitted by diode lasers will increase as the temperatures of their crystal structures rise, causing sudden jumps in wavelength, or "mode hopping." In between mode hops, lasing wavelengths will vary linearly with laser temperature.

 

Apart from mode hopping, thermal drift in lasing wavelength will amount to about 0.2 nm/°C.

 

Lasing wavelengths will also vary with output power, since varying drive current alters diode temperature.

 

 

 

 

Service life

 

 

Diode lasers are reliable, long-lived components. Given proper care and handling, they have virtually unlimited service lives. Their service lives will depend upon:

 

• Operating temperatures
• Drive currents
• Handling 

 

Both current/voltage spiking and electrostatic discharges can damage diode lasers. Appropriate protective measures (active and passive startup-transient suppression) protect LINOS Photonics diode lasers against:

 

• Current and voltage spiking
   - arising from switching transients
   - transmitted by electrical supply lines
   - induced by nearby AC power lines or other circuitry
• Electrostatic discharges