Using the current geometry of our unibody laser, a range of wavelengths is accessible for a given diffraction grating. From the master gratings listed by Newport's Richardson Gratings division we have compiled the following plots to visualize the collective region of functionality for our design.
Situations which require a large deflection of the grating arm can be mitigated by using a shim behind the grating on the end of the arm. This prevents the arm flexure from being over stressed and fatigued (by deflection of more than about 2 degrees away from 45), and also allows the reflected beam to be appropriately adjusted by anamorphic prisms and directed to the fiber coupler. Wavelengths which require such a shim are indicated by the blue color, while wavelengths which require no shim are in red.
Warning: These figures do not include an investigation into available diodes. Geometry supporting a particular wavelength does not imply a laser diode can emit that wavelength!
The unibody design was put together with the intent that all lasers shared
the exact mechanical specifications. Thus, the only considerations to
address after selecting a desired wavelength are: the diode itself, the
diffraction grating, the placement of the anamorphic prism pair, and the
output fiber. Some simple calculations are required, but outlined in the
details below.
DIODE: Thorlabs, Sacher Lasertechnik, Eagleyard Photonics, Meshtel, and
qPhotonics are typical vendors to get diodes from. Keep in mind that while
expensive, an AR coating will allow a diode's wavelength to be pulled
further by the grating feedback and that the laser's linewidth will be
narrower. Note the divergence angles of the diode to be used later in the
anamorphic prism pair calculations.
GRATING: Newport (Richardson gratings) has by far the most variety in their reflective holographic gratings. Since we use Littrow configuration, the grating equation is $\lambda = 2d sin(\theta)$, where $\theta$ is the angle of incidence of the beam with respect to the grating's normal (this is equal to the diffraction angle of the first order...hence the factor of 2). For the wavelength in question, try some of the different values of $d$ offered by the grating vendor. $\theta$ should be as close to $\pi/4$ as possible so that the angle between the cavity and the output beams is nearly right.
This will not be perfect, but the idea is that we can shim the grating and calculate where to place the prisms to produce a shaped beam which exits the laser body at the fiber coupling point. In the script used to calculate the prism locations, it is assumed that the shim will be on the edge of the grating which is at the end of the adjustable arm. Therefore, $\theta$ should be as close to $\pi/4$ as possible WITHOUT exceeding it (that is, the total between input and reflecting beams is nearly right, but acute). So far (with wavelengths run calculations for), if the angle between incident and 0th order reflection is different from 90 degrees by less 7 degrees, the prisms can appropriately compensate the altered beam path while remaining inside the 'prism pocket'.
Shim note: we've been able to get away with leaving out the shim for shim
angles less than 2 degrees by adjusting the grating arm a bit farther than
its intended location. This allows us to skip the somewhat tricky step of
gluing the grating on with a shim, and the only penalty we pay is a little
deviation from optimum prism location. (The prism guide is still good
enough to have a circular beam and couple to a single mode fiber, though!)
ANAMORPHIC PRISM PAIR: The octave script anamorphic.m calculates the appropriate positions of the prism pair so that the beam exiting the laser has the correct shape (uses parameters from diode specifications), and exits at the correct location and angle. For a new wavelength, the code must be altered accordingly.
The code checks if reflections off the prisms are possibly problematic and
reports small reflection angles if they occur. It generates an .eps file
that can be printed and placed as a guide within the prism pocket in the
main body. We used a thin sheet of acrylic and etched the guide using a 30W
laser cutter.
Double check if desired grating-wavelength combination works with design's current geometry, and calculate the thickness of shim you'll need:
Desired Wavelength:nm