The output frequencies of a laser are determined by several factors. First, the gross wavelength is determined by the energy uncertainty (broadening) of the laser transition, which determines the wavelength and overall linewidth. Nonetheless, at any given instant, only a relatively few frequencies within this overall envelope are allowed to oscillate. These "longitudinal modes" result from the boundary conditions that, in a conventional two-mirror lasers, the amplitude of the wave must be zero at the mirror surface (i.e., that the oscillating wave is a standing wave). This means only those laser frequencies that meet the criterian = Nc/2L

can operate, where c is the speed of light, L is the effective cavity length, and N is an interger. Adjacent modes are typically orthogonally polarized.

The illustration below shows the lasing envelope of a helium neon laser operating at 632.8nm with a cavity spacing of 23cm. This results in a mode spacing of 640MHz. Since the width of the gain curve (FWHM) is only 1400MHz, only two longitudinal modes can operate at any given time. If the laser were twice as long, four longitudinal modes could operate simultaneously.

Since the allowable longitudinal modes are a function of cavity length, the frequency will change as the cavity length changes. In lasers where only a few longitudinal modes can operate, these changes will cause outpout power to fluctuate as the modes sweep under the gain curve

The human eye can see light in the visible spectrum ranging from Red to Violet, i.e. the colours of the rainbow. The eye's sensitivity is non linear, so different colours are perceived at different intensities to the eye. So for the same given power, a 100mW Red laser will appear much dimmer than a 100mw Green laser.The colour the human eye is most sensitive to is a shade of Green which has a wavelength of 555nm.

Wavelength  Response   Color           Typical Source/Applicatio

-------------------------------------------------------------------------------
   350 nm    .00001   UV

   380 nm    .0002    Near UV

   400 nm    .0028    Border UV      Nichia violet GaN laser diode

   410 nm    .0074     "                  "

   420 nm    .0175    Violet
   442 nm    .0398    Violet-blue    Violet-blue line of HeCd laser

   450 nm    .0468    Blue
   457.5 nm  .0556     "             Blue frequency doubled Nd:YVO

   457.9 nm  .0562     "             Blue line of argon ion lase

   473 nm    .104      "             Blue frequency doubled Nd:YAG

   488 nm    .191     Green-blue     Green-blue line of argon ion laser

   500 nm    .323     Blue-green
   510 nm    .503     Green          Emerald green line of copper vapor laser
   514.5 nm  .588      "             Green line of argon ion laser

   532 nm    .885      "             Green frequency doubled Nd:YAG or Nd:YVO

   543.5 nm  .974      "             Green HeNe lase

   550 nm    .995     Yellow-green
   555 nm   1.000      "             Reference (peak) wavelength
   567 nm    .969      "             Green line of Helium-Mercury laser

   568 nm    .964      "             Y-G line of some krypton ion lasers

   578 nm    .889     Yellow         Gold line of copper vapor laser

   580 nm    .870      "

   594.1 nm  .706     Orange-yellow  Yellow HeNe laser

   600 nm    .631     Orange

   611.9 nm  .479     Red-orange     Orange HeNe laser

   615 nm    .441      "             Orange line of Helium-Mercury laser

   627 nm    .298      "             Orange line of Gold Vapor Laser

   632.8 nm  .237     Orange-red     Red HeNe laser

   635 nm    .217      "             Laser diode (DVD, newer laser pointers)

   640 nm    .175      "                 "

   645 nm    .138      "                 "

   647.1 nm  .125     Red            Red line of krypton or Ar/Kr ion laser
   650 nm    .107      "             Laser diode (DVD, newer laser pointers)

   655 nm    .082      "             Laser diode

   660 nm    .061      "                 "

   670 nm    .032      "             Laser diode (UPC scanners, old pointers)

   680 nm    .017      "

   685 nm    .0119    Deep red

   690 nm    .0082     "

   694.3 nm  .006      "             Ruby laser

   700 nm    .0041    Border IR

   750 nm    .00012   Near IR

   780 nm    .000015     "           CD player/CDROM/LaserDisc laser diode

   800 nm    3.7*(10)-6  "           Laser diodes for pumping Nd:YAG, Nd:YVO

   850 nm    1.1*(10)-7  "

   900 nm    3.2*(10)-9  "
   1,064 nm  3*(10)-14   "           Nd lasers (including YAG)
   1,523 nm  0.0000      "           IR HeNe laser

   3,390 nm  0.0000     Mid-IR       IR HeNe laser

   10,600 nm 0.0000     Far-IR       CO2  laser

                      

The DPSS Lasers of 532nm is very close to the peak of the eye's sensitivity. The greens and blues produced by a typical argon are further away from the peak response of the eye, so they appear to be dimmer in comparison. This is why DPSS laser are said to be 3 to 4 times brighter than a typical argon laser. So a 100mW DPSS laser will look the equivalent to a 300mw argon laser in terms of apparent brightness.

Taking this to an extreme, if you were to compare a 100mw 532nm Diode laser with a 2W Infrared Laser e.g. emitting 1064nm, the 100mW laser will look much brighter because the human eye can't see infrared wavelengths!