633 nm direct diode lasers are substitutes for previously popular HeNe lasers. Diode lasers have advantages of small size, higher output power and long operational time while still maintaining stable single frequency operation. Furthermore, these lasers offered by Integrated Optics have much better pointing stability and polarization contrast. These two features eliminate the most significant drawbacks of HeNe lasers in applications where coupling to a single-mode polarization-maintaining fiber is needed. Large HeNe laser tubes were subject to strong change in physical properties upon change of environmental temperature. This is not the case with diode lasers.