In the past decade, diode lasers have made great strides in both power and reliability. The performance improvements have enabled new applications, particularly in areas where optical brightness is a key performance metric. The fiber-coupled diode pumping of fiber lasers is a critical technology in commercial, industrial and defense applications. Materials processing, once the singular province of fiber lasers, can now be addressed in some respects by diode laser beams directly delivered by optical fiber.
Diode lasers are typically constructed of molecularly layered binary, ternary, and quaternary semiconductor crystals grown by molecular beam epitaxy or metal organic chemical vapor deposition. The job of the diode laser engineer is to properly select the constituent elements and structure the layers to achieve the requisite electro-optical properties of a diode laser – efficient control of the flow and capture of electrons and the proper guiding of the laser light through the structure.
Semiconductor crystals are deliberately grown to incorporate impurities into the lattice. These impurities serve to control the conductivity of the semiconductor, with one side of the structure receiving extra electrons and the other side positive carriers known as holes. Near the center of the diode laser structure are quantum wells which serve as the diode laser’s active region. The quantum wells are only several tens of angstroms across, narrow enough to create quantized energy states for both the electrons and holes. The energy difference between the quantized energy states of the carriers determines the diode laser’s emission wavelength. Careful engineering of the strain in the quantum wells allows the engineer to further tailor such properties as the threshold current and the gain characteristics of the device.
When the epitaxial structure is processed, metalized layers deposited on the semiconductor surface serve as the injection area of the electrical carriers that, upon transport in sufficient quantity to the active region, create the population inversion necessary to achieve lasing. The lasing structure is completed by cleaving either side of the semiconductor to create the reflective facets needed for the final element necessary to achieve lasing, that of optical feedback.
In practice, the facets of the diode laser are coated with dielectrics that enhance reflectivity at one end and diminish it at the other, ensuring most of the light exits one end of the laser. The cleaving of the semiconductor to create the facets leaves dangling atomic bonds at the crystal/air interface, which serve as electrical carrier recombination sites, creating heat rather than light. The facets have historically been the limiting factor in achieving both long life and high power from diode lasers with the facet failing due to catastrophic optical damage enabled by the carrier recombination. Advances in treatments and coatings at the facets are responsible for the rapid advance in recent years of maximum achievable power and concomitant improvement in reliability that can now extend to tens of thousands hours of usable life. Figure 1 depicts representations of the diode structure and optical cavity of a diode laser.
In the plane normal to the diode laser plane, the coherent optical field generated through the recombination of the electrons and holes in the active region is guided by the optical index variation introduced by the various layers of the semiconductor. It is one of the rare, fortuitous gifts of nature that the same semiconductor layers that guide the laser optical field so well also have the characteristic of confining the electrons where they need to be confined in the diode. As the optical guiding layers are very thin, just a few microns in depth, and the confinement is diffraction limited, this axis of the optical field diverges very rapidly, oftentimes in the range of more than 80 degrees for full width at 90% power enclosure. As such, it is referred to as the fast-axis of the diode.
Parallel to the plane of the diode, the optical field is either very weakly optically guided or guided only by the region of the semiconductor where sufficient injection current exists to achieve optical gain. The width of the injected region as defined by the metallization is typically 50 to 200 microns. Given such a broad, optically multi-mode profile, the light emitted in this direction is far from diffraction limited and is typically referred to as the slow-axis of the device. Divergence in the slow-axis is typically 10-15 degrees at full-width, 90% power enclosure.
It is the properties of the fast- and slow-axes that the optical engineer must manipulate to efficiently couple the laser light into an optical fiber. A characteristic of the diode is its brightness – the ratio of the output power to the beam parameter product (BPP), where the BPP is the product of the beam waist size and its divergence. The brightness of the diode laser is at best a conserved quantity, meaning it can’t be improved upon with only passive means, but that it can be diminished through power loss and beam distortion. The art of high brightness fiber-coupled diode laser module design is the development and manufacture of the highest brightness diode lasers possible and then manipulating the laser light into the optical fiber while losing as little of the original brightness as possible.
The emitted laser light must be rapidly collimated after exiting the diode facet so that it is not lost through absorption or scattering in the module. The fast-axis is typically captured by a cylindrical lens of 500-1000 micron focal length. The diffraction limited nature of this axis results in a residual fast-axis divergence in the low, single digits of milliradians.
In the slow axis, each diode laser emitter on the laser bar has its own slow-axis lenslet. To achieve best collimation it is desirable to fill as much of the lenslet aperture as possible, resulting in scattering losses at the edges where each of the lenslets abut. Due to the multi-moding and a beam well above the diffraction limit, these lenslets can only achieve a residual divergence of 10-15 milliradians while keeping losses in check. Other sources of power loss and, hence, brightness are losses at optical interfaces and through non-ideal lens shapes.
A common measure of the optical quality of light is M2. An M2 of 1 means the beam is a perfect Gaussian in that axis. Any increase in the value M2 in excess of 1 means an ever worsening beam quality. For diode lasers, the fast-axis is reasonably close to a M2 value of 1 but the slow axis is closer to 10 or more for each emitter. It is therefore desirable to transform the beam such that one axis retains the poor M2 of just a single beam in the slow axis direction, while stacking beams in the other axis such that the near ideal fast axis from each emitter is additive. The net result is a much more symmetric beam quality profile that can more easily be coupled into the optical fiber.