  The unique optical properties of phase change materials (PCMs) can be
exploited to develop efficient reconfigurable photonic devices. Here, we
design, model, and compare the performance of programmable 1X2 optical couplers
based on: Ge$_2$Sb$_2$Te$_5$, Ge$_2$Sb$_2$Se$_4$Te$_1$, Sb$_2$Se$_3$, and
Sb$_2$S$_3$ PCMs. Once programmed, these devices are passive, which can reduce
the overall energy consumed compared to thermo-optic or electro-optic
reconfigurable devices. Of all the PCMs studied, our ellipsometry refractive
index measurements show that Sb$_2$S$_3$ has the lowest absorption in the
telecommunications wavelength band. Moreover, Sb$_2$S$_3$-based couplers show
the best overall performance, with the lowest insertion losses in both the
amorphous and crystalline states. We show that by growth crystallization tuning
at least four different coupling ratios can be reliably programmed into the
Sb$_2$S$_3$ directional couplers. We used this effect to design a 2-bit
tuneable Sb$_2$S$_3$ directional coupler with a dynamic range close to 32 dB.
The bit-depth of the coupler appears to be limited by the crystallization
stochasticity.
