They aren't any particular formulas particularly for He-Ne lasers, but i think some of the below points may help you:
#rArr# In He-Ne lasers, the ratio of He-Ne is in the order 10:1 respectively.
#rArr# The wavelength of emission of He-Ne laser is 632.8nm.
#rArr# Usually it is constructed to operate in the red light at 6328 #A^o#.
However, it can be constructed to produce laser action in the green light at 5435 #A^o# and in the infrared at 15,230 #A^o#.
#rArr#Beam Divergence Angle #theta=(lambda)/(pir_o)#
where #lambda# = wavelength of laser beam
#r_o# = internal beam radius.
#rArr# Intensity #I=(P)/(A)# where #P# = Power(in Watts) and #A# = Area .
#rArr# He-Ne lasers has got very low power about 0.5#-#5mW.
#rArr# Coherence length of the beam #L_c = (c)/(Deltanu)#
where #Deltanu# = uncertainity in the frequency of the laser beam #=(1)/(tau_c)# and #tau_c# is the coherence time.
#rArr#degree of non-monochromaticity #= (Deltanu)/(nu_o)#
where #nu_o = (c)/(lambda_o)#.
