How Do Newly Available Green Diode Lasers Compare to Diode-Pumped Solid-State Lasers?

GaN based green laser diodes began appearing commercially about 5 years

ago, and typically operate around 520 nm. 530 nm devices may also be

purchased, but at a substantially higher price. As shown in the Figure above, 520

nm laser diodes are further away from the peak human eye photopic peak

sensitivity, and compared with 532 nm DPSS lasers, are 30 % less visible. APSci

has tested Osram green laser diodes operating at 520 nm with our DPSS

MicroGreen and MiniGreen lasers operating at 532 nm, at the same output

power level, and the green Osram diodes appear distinctly blue-green, whereas

our DPSS lasers appear as a true green color. A comparison of the operating

characteristics reveals that Osram green laser diodes actually have a lower

operating wallplug efficiency that our DPSS lasers. In addition, the Osram green

diodes require an operating voltage of typically 6.5 – 7 V, significantly higher

than the typical 2.0-2.2 V required by DPSS lasers. Thus off the shelf laser diode

drivers cannot be used, necessitating the development of new power supplies,

particularly miniature drivers needed for compact modules. In addition to the

aforementioned disadvantages, green diodes, like must modern diode lasers,

have a highly astigmatic power output, where the fast axis perpendicular to the

stripe is substantially Gaussian and diffraction-limited with a large 1/e 2 beam

divergence, whereas the slow-axis parallel to the stripe is highly multimode

with a much lower 1/e 2 divergence. This behavior makes collimation of the

output beam and the achievement of uniform illumination difficult to obtain. In

contrast, a DPSS green laser emits a near-diffraction- limited circularly

symmetric Gaussian beam that is easy to collimate and that propagates with a

predictable circular profile.

In spite of the obvious disadvantages of green diode lasers, for some

applications their use is warranted. Green diodes are less temperature sensitive

than DPSS green lasers, and for cases like laser pointing where a bright spot is

required, are simpler to deploy. The brightness of the spot size masks the

underlying poor beam profile thanks to the saturation of the response of the

human eye. In other applications such as line generators that use a Powell lens,

orienting the diode can result in a perfectly acceptable uniform green line. In

general, applications involving distances of some tens of meters can be

successfully addressed with green laser diodes, whereas green DPSS lasers are

ideal for further distances out to many kilometers. It should be pointed out,

however, that for applications like laser Dazzlers (ocular interrupters) in which

eye-safe performance is a necessity, it is far easier to build such devices using

predictable circularly-symmetric DPSS green beams than the multimode

astigmatic beams produced by green laser diodes.