- Volumetric Bragg Grating Dramatically Cuts Laser Bandwidth
The inherently broad bandwidth of solid-state lasers poses a problem for many applications — for example, spectroscopy and many other scientific applications, sensors and nonlinear optics — where nearly perfectly monochromatic light is desirable. Researchers at the University of Central Florida in Orlando recently demonstrated a straightforward technique to reduce the bandwidth of solid-state lasers by two to three orders of magnitude, with negligible accompanying loss in output power.
Because solid-state lasers are homogeneously broadened, the gain that normally is spread across the lasing bandwidth can be equally effective in a narrow band if the resonator’s feedback is restricted to that band. Many techniques have been developed to restrict the bandwidth of a resonator’s feedback, including intracavity etalons, metallic films, birefringent filters, prisms and surface Bragg gratings. These elements, individually or in combination, have proved effective in reducing the bandwidth of lasers, but because they introduce an intracavity loss, all significantly lower the output power.
Volumetric Bragg gratings recorded in photothermally refractive glass are robust optical elements that have been shown to be effective in reducing the bandwidth of diode lasers. These gratings can act as transmissive or reflective narrowband filters, diffracting in excess of 99 percent of the light in a narrow, ~100-pm band. When substituted for one of the laser’s mirrors, a grating reduces the bandwidth of the resonator’s feedback without introducing the loss associated with other intracavity elements.
Figure 1. The researchers substituted a volumetric Bragg grating for the output coupler of a Ti:sapphire laser. The grating diffracted ~99.8 percent of the light passing through it back along the incoming direction. But its glass surfaces were uncoated, resulting in 4 percent Fresnel reflection from each surface; these reflections provided the output coupling for a multibeam output. A more useful, single-beam output could be readily obtained by applying an antireflection coating to the glass grating and using it as the back mirror, or by adjusting the diffraction efficiency and using a coated grating as the output coupler. Images ©OSA.
The researchers in Florida replaced the output couplers of both Ti:sapphire and Cr:LiSAF lasers with vol-umetric Bragg gratings (Figure 1). They measured the lasers’ bandwidths with a spectrometer from Ocean Optics Inc. of Dunedin, Fla., and a Fabry-Perot interferometer. They first characterized the Ti:sapphire laser when its output coupler was a normal dielectric mirror and observed a full width half maximum bandwidth of 0.5 to 1.0 nm, with significant additional structure such as shoulders and sidebands. But when they substituted the volumetric Bragg grating, the bandwidth shrank to about 2.5 pm, corresponding to oscillation of no more than three or four longitudinal modes.
The investigators obtained similar spectral narrowing in a Ti:sapphire laser with a dielectric mirror output coupler when they inserted a three-plate birefringent filter into the resonator. But the diminishment of the output power in this case was dramatic: Although a pump power of 2 W resulted in almost 600 mW from the 2.5-pm-wide grating-coupled laser, the same pump power barely pushed the birefringent-filtered, 2.5-pm-wide laser above threshold and produced a meager ~10 mW (Figure 2).
Figure 2. The output of the Ti:sapphire laser with the volumetric grating output coupler (green) was essentially the same as that of the same laser with a dielectric mirror output coupler (purple). But the unrestricted bandwidth of the laser with the dielectric coupler was 0.5 to 1.0 nm, while the grating-coupled laser’s bandwidth was 2.5 pm. A similar 2.5-pm bandwidth could be obtained from the laser with the dielectric coupler by adding an intracavity birefringent filter, but only with a significant reduction of output power (red).
To demonstrate the versatility of the volumetric Bragg grating, the researchers also substituted it for the output coupler of a flashlamp-pumped Cr:LiSAF laser. They observed a spectral narrowing by a factor of ~2000, while the output energy remained well over half that with the dielectric coupler.
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