Both TVBGs and RVBGs are widely used in photonics and lasing thanks to their exclusive properties. One of the most important applications of VBG is combining of several beams produced by diode, fiber or disk lasers. One approach for beam combining is spectral beam combining (SBC). In order to combine two laser beams the first one can directly pass through the grating while the second beam should un-dergo Bragg reflection [65, 66]. The direction of the first beam is chosen so that it is co-linear with the first diffraction order of the grating. In this configuration the out-put beam is not coherent, because inout-put beams are produced by independent laser sources. Moreover, the spectral width of output beam is proportional to number of combined lasers. The wavelength of each laser source should be calculated taking into account the reflection spectrum of VBG in use. More than two laser beams can be combined using several separate substrates with single VBG or by multiplexing of several VBGs fabricated in the same volume of the PTR glass [3, 16, 65, 67, 68].
Figure 2.9: Multi-channel fiber laser coherent locking with VBG [2]
Specifically, each grating is recorded for a particular Bragg angle in such a way that all diffracted beams propagated in the same direction. VBGs can be also used for coherent beam combining (CBC) [2]. In this approach, laser beams from two or more laser sources are reflected from multiplexed VBGs recorded in PTR glass to mutual output resonator mirror for both laser sources. The output power from the mirror is coherent. The wavelength of each laser should be the same providing narrow spec-tral width of output radiation. Both approaches allow one to generate high quality laser beams with power of the order of tens of kilowatts [65].
In past decades for high-power CW and pulsed sources for pumping solid-states lasers needed for laser cutting, drilling, soldering etc are of high demand. Fiber, thin-disk, diode and CO2 lasers take the largest part of the market of the high-power materials processing. Laser diodes, which are widely used for the pumping of solid state lasers, have spectral width of several nanometers. It causes extra heat-ing of active media due to photon-phonon couplheat-ing. That is the spectral width can be significantly reduced by laser stabilization with VBG as output mirror of exter-nal diode laser resonator [70–74]. Whereas leaking longitudiexter-nal mode has narrow bandwidth the rest of the modes are accumulated in the resonator and eventually couple into the leaking mode. Thus, the output power drop can be as small as 10%
from initial value. In similar manner RVBGs have been demonstrated to stabilize fiber, hybrid and bulk crystal laser in CW and pulsed regimes allowing one to to achieve a single mode operation with line width as narrow as several pm [2, 75–88].
The use of VBG also allow to make these lasers tunable in wide range up to 50 nm
Figure 2.10: Two BragGrateTMNotch filters (BNFs) and thin film notch filter (blue) optical density spectra. The right inset shows magnified spectral region of a 785 nm BNF. [69]
by rotation of the grating which changes Bragg condition [75, 82, 89–96].
Narrow and tunable spectrum of stabilized laser makes it possible to combine the laser beams in coherent or non-coherent way as it was described before [2,67,68,97].
Stabilized with VBGs laser diode bars are used for zero-phonon line pumping of thin-disk laser with following decrease of active medium heating up to 32% [98–100].
Another important feature of laser stabilization is that it allows to reduce a shift of central wavelength caused by heating of diode laser at higher current. The use of VBG as external mirror prevents wavelength drift because Bragg condition is determined strictly by the design of the grating.
VBG found an application in beam quality improvement as a spatial filter. The grating can be placed both to the laser resonator for selection of low index trans-verse modes or outside the laser resonator [101]. The mode selection is achieved due to high angular selectivity of transmissive VBGs, which diffracts only beams propagating in very narrow cone of angles [50]. At the same time the diffraction efficiency can be up to 90-95% and loses induced by the grating are small enough due to low absorption and scattering and high homogeneity.
Reflective VBGs are also in wide use for Raman spectroscopy [69]. Raman signal frequency is usually very close to the laser line and has relatively low intensity. The laser line has to be suppressed by several orders of magnitude. Notch-filters based on RVBGs allow to detect Raman signal with 3-4 cm−1with 10-40 dB attenuation of laser line (Fig. 2.10). Moreover, a detection of anti-Stocks shift became possible as well with RVBGs because it works selectively for a particular wavelength.
Refractive VBGs are used for chirping of ultrashort pulses instead of well-known chirped pulse amplification (CPA) [75,102]. A grating with gradually varying period can stretch and compress ultrashort pulses depending on direction of propagation.
Instead of two collimated beams interference one can use interference of diverging and converging beams with equal angles to achieve linear change of grating period.
Each wavelength of transfer-limited pulse reflects on the grating with certain delay.
chirped VBGs recorded in PTR glass can be used in the range 0.8 to 2.5 um with diffraction efficiency up to 90 %. Nowadays femtosecond pulse can be stretched up to 1 ns duration and compressed back to 200 fs with energies and average power 1
Figure 2.11: Scheme of RVBG (a) and chirped VBG (b) [102].
mJ and 250 W respectively. The main drawback of such chirping approach is losses induced by the grating during the pass which can be as high as 20% per double pass.