technology nitride lasers
rhombus4
Alternative approaches to reducing the internal built in possible to produce directly emitting green lasers at
piezoelectric fields are based on semi-polar or non-polar wavelengths of 515nm and beyond on polar and semi-
GaN substrates. However, lasers grown on non-polar m- polar planes. Future goals include improvements in the
plane GaN substrates currently suffer from poor material material quality of the quantum wells. If success follows,
quality of the high indium-content layers needed to small laser chips could replace the bulky, complex devices
produce green emission. currently used to provide a green laser source.
Although there are issues affecting all forms of nitride ring6 The German Federal Ministry for Education and Research
laser, this should not obscure the tremendous progress (BMBF) supports the research activities on blue and green
made with green lasers over the last year or so - it is now InGaN lasers within the project MOLAS (FKZ 13N9373)
Applications served by green lasers
With a photopic response of about image can be displayed on arbitrary
600lm/W, the green wavelength is ideal for surfaces without focusing. Further
applications where a laser beam or beam advantages of laser based projection
pattern is used to visually align, level or displays are high system efficiency, because
position. In comparison, commonly used lasers emit polarized light and have, by
red laser diodes have a significantly inferior nature, a very low etendue. Optics can also
photopic response, and this that means be less complex compared to other light
that roughly four times the laser output sources enabling significantly smaller display
power is necessary to achieve the engines. Due to the monochromaticity of
equivalent level of brightness of a green lasers the color gamut is superior to any
laser, which is a major impairment to eye other projection technology.
safety. Although high absolute output Implementations of laser projection displays
power is not that critical in these can be found in pico projectors and rear-
applications, a large operating temperature Embedded laser projector with red, green projection TVs, and they will soon appear in
range, high efficiency, ruggedness and and blue lasers. Credit: MVIS head-up displays in cars.
simple control are key requirements,
especially for portable, battery-powered leveling or positioning in
construction and industrial environments. Medical treatments
Green lasers are ideal for a wide variety of medical applications
Color displays because this wavelength is absorbed by oxyhemoglobin and
Green lasers are also an essential component in any form of full melanin, and by pulsing this source it is possible to ensure that
color projector. Here red, blue and green lasers are used together thermal damage is confined to the targeted tissue or vessel by
to either directly scan a picture, or to project a picture by selecting a pulse length and duty cycle that enables the targeted
backlighting a micro display. If a micro-display approach is used, area to cool down between pulses.
the imager is sequentially illuminated by these three laser sources
and the monochromatic picture (color by color) is projected through In Europe and North America the most common cause for
an optical system. Usually it takes 4 ms to produce a single frame, blindness is diabetic retinopathy, which results from diabetes
which is one monochromatic picture, so it follows that the laser mellitus, and can be controlled with laser eye treatment. This eye
must be modulated in the kHz range. condition is caused by a lack of oxygen in the retina, which stems
from vessel damage associated with poorly controlled blood-sugar
Another display technology is the laser scanning beam projector, levels. Unwanted new blood vessels can then start to spread onto
which is based on a fast deflecting MEMS-based micro mirror. The the retina and into the vitreous humour - the clear gel that fills the
system works by scanning the entire picture a pixel at a time and space between the lens and the retina of the eyeball - but this can
modulating the laser directly at 10-100 MHz, much like the electron be prevented by treating the eye with a green laser.
beams in a cathode ray tube used in a home TV. “Gray scales” are
realized by amplitude modulation of the lasers. Lasers with excellent A technique known as panretinal laser coagulation is used on a
beam quality are needed to serve this type of projector, due to the large area of the retina to create 1000-3000 burn spots that can
small diameter of the micro mirror and the influence of the spot size reduce the overall oxygen demand, thus supporting the still healthy
on picture quality. No conventional imaging optics is needed, so the parts of the retina.
Further reading: [6] T. Miyoshi et al. Appl. Phys. Express 2, 062201 (2009).
[1] T. Miyoshi et al. Proc. SPIE 6894, 689414 (2008) [7] K. Okamoto et al. Appl. Phys. Lett 94, 071105 (2009)
[2] D. Queren et al., Phys. Status Solidi C 6, No. S2 (2009). [8] Y. Enya et al. Appl. Phys. Express 2, 082101 (2009)
[3] D. Queren et al. J. Cryst. Growth 311, 2933 (2009). [9] Y. Yoshizumi et al. Appl. Phys. Express 2, 092101 (2009)
[4] D. Queren et al. Appl. Phys. Lett. 94, 081119 (2009). [10] Stephan Lutgen et al. ICNS-8 2009, to be published in Phys
[5] A. Avramescu et al. Appl. Phys. Lett. 95, 071103 (2009). Stat. Sol.
24
www.compoundsemiconductor.net January/February 2010
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87