High power diode laser bars are interesting in many applications such as solid state laser pumping, material processing,
laser trapping, laser cooling and second harmonic generation. Often, the free running laser bars emit a broad spectrum of
the order of several nanometres which limit their scope in wavelength specific applications and hence, it is vital to
stabilize the emission spectrum of these devices. In our experiment, we describe the wavelength narrowing of a 12
element 980 nm tapered diode laser bar using a simple Littman configuration. The tapered laser bar which suffered from
a big smile has been "smile corrected" using individual phase masks for each emitter. The external cavity consists of the
laser bar, both fast and slow axis micro collimators, smile correcting phase mask, 6.5x beam expanding lens
combination, a 1200 lines/mm reflecting grating with 85% efficiency in the first order, a slow axis focusing cylindrical
lens of 40 mm focal length and an output coupler which is 10% reflective. In the free running mode, the laser emission
spectrum was 5.5 nm wide at an operating current of 30A. The output power was measured to be in excess of 12W.
Under the external cavity operation, the wavelength spread of the laser could be limited to 0.04 nm with an output power
in excess of 8 W at an operating current of 30A. The spectrum was found to be tuneable in a range of 16 nm.
High power diode lasers are used in a large number of applications. A limiting factor for more widespread use of broad
area lasers is the poor beam quality. Gain guided tapered diode lasers are ideal candidates for industrial applications that
demands watt level output power with good beam quality. By adapting a bar geometry, the output power could be scaled
even up to several tens of watts. Unfortunately, the high divergence which is a characteristic feature of the bar geometry
could lead to a degradation of the overall beam quality of the laser bar. However, spectral beam combining is an
effective solution for preserving the beam quality of the bar in the range of that of a single emitter and at the same time,
enabling the power scaling. We report spectral beam combining applied to a 12 emitter tapered laser bar at 980 nm. The
external cavity has been designed for a wavelength separation of 4.0 nm between the emitters. An output power of 9 W
has been achieved at an operating current of 30 A. The combined beam had an M2 value (1/e2) of 5.3 along the slow axis
which is comparable to that of a single tapered emitter on the laser bar. The overall beam combining efficiency was
measured to be 63%. The output spectrum of the individual emitters was narrowed considerably. In the free running
mode, the individual emitters displayed a broad spectrum of the order of 0.5-1.0 nm while the spectral width has been
reduced to 30-100 pm in the spectral beam combining mode.
The high energy efficiency and radiant flux of high-power LED devices has lead to many new applications of LED
lighting. Within the more production oriented applied plant research, there is a need for illumination systems that ensures
a high irradiance, spectral control and homogeneous illumination of a large plant canopy to ensure reproducible results
over long term measurements.
A new high power LED illumination system is presented. It has been designed and developed for illumination of a plant
canopy area of 60 x 80 cm2 in a climate chamber where photosynthesis of the whole canopy can be measured. The LED
system extends the precise control of the chamber climate with computer control and long term stability of the irradiance
and spectral composition of the illumination. High-power red and blue (at 455 and 639 nm) LED devices have been
chosen that coincides with the absorption peaks of chlorophyll. The illumination system allows for a maximum
irradiance of 6.3 W/cm2 corresponding to a photosynthetic photon flux density (PPFD) of 300 μmol m-2 s-1. The spectral
composition of the light given by the ratio of blue photons compared to the total number of red and blue photons can be
adjusted from 0-40 % keeping the irradiance at a constant level. Spectroradiometric 2D grid measurement at the plant top
level shows homogeneity of ± 5% of the irradiance and ± 5% of the spectral distribution, over almost the entire canopy
area. Initial experiments carried out on Chrysanthemum plants showing the dependence of the photosynthesis on blue
light fraction is presented and discussed.
KEYWORDS: Semiconductor lasers, Mode locking, Laser systems engineering, Near field, Diodes, Beam splitters, Broad area laser diodes, Near field optics, Laser applications, Optical testing
In this paper, the experimental results of self-injection phase locking in a 10 W, single element wide broad-area diode laser are presented. The width of the emitting area of this diode is 1000 μm, to our knowledge it is the broadest single element diode laser that until now has been used in an external feedback cavity. The beam quality of the diode laser is improved by the asymmetric self-injection phase locking technique. The far-field and near-field profiles are measured with and without the self-injection phase locking at different operating currents. 3.65 W output power is obtained with the operating current of 13.0 A, with a beam quality factor M2 of 16.6, which is improved with a factor of 14 by the self-injection phase locking technique.
In this paper, we demonstrate lateral mode selection and amplification in a broad area laser (BAL) diode in an external cavity. The cavity is based on self-injection locking of an 807 nm, 3W broad area diode using a mirror stripe as the feedback unit. At the optimum mirror stripe position, the lateral far-field intensity profile is narrowed 8.5 times compared with the profile from the freely running laser when running at a drive current of twice the threshold current. We have determined the lateral angular range, in which, different array modes can be exited and, only, within a narrow range around 2.3° from the beam center a high, spatial beam coherence can be obtained.
The beam quality of a diode laser with two active segments was improved by an external cavity. The cavity includes collimating optics, a grating, and an output coupler. The beam quality was improved by a factor of 2, and at least half of the freely running power of the laser was coupled out from the external cavity.
The power from two similar diode laser systems with a center wavelength of 810 nm was added via polarization coupling. Each system consisted of a 1 μm x 200 μm broad area laser diode applied with external feedback. A power of 1.2 W, corresponding to 40% of the freely running system, with an M2 value of 1.5 were obtained in the coupled output beam when operating the diodes at a current of three times the threshold current.
It is possible to quantify cutting of plant stems by a laser beam in contrast to mechanical cutting. The biomass of the plants after a certain period under standard green house conditions was used to measure the effect of partial or complete cutting with a laser. Continuous laser irradiation at 10.6 micrometers of the plant stem turned out to be very efficient at values of the energy per width unit above 6 J/mm. The effect of laser irradiation at 355 nm or 1064 nm is less pronounced, but also at these wavelengths the re- growth or continuous growth are reduced. A monocotyledon- type, winter what (Triticum vulgare L), is substantially more resistant than a dicotyledon-type, charlock (Chenopodium album L.) against radiation. The exposure limits for laser light in living plants have been explored as well. The limit in terms of re-growth of the irradiated plants exceeds the MPE (maximum permissible exposure) of human skin by several orders of magnitude. The consequence is that very powerful (unfocused) lasers can be used in any environment without significant impact on living plants.
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