Intracavity frequency-doubled degenerate laser

Abstract

We develop a green light source with low spatial coherence via intracavity frequency doubling of a solid-state degenerate laser. The second harmonic emission supports many more transverse modes than the fundamental emission, and exhibit lower spatial coherence. A strong suppression of speckle formation is demonstrated for both fundamental and second harmonic beams. Using the green emission for fluorescence excitation, we show the coherent artifacts are removed from the fullfield fluorescence images. The high power, low spatial coherence and good directionality makes the green degenerate laser an attractive illumination source for parallel imaging and projection display. PACS numbers:Visible lasers have found a wide range of applications in imaging, spectroscopy and display. Their brightness, power efficiency and emission directionality make them more suitable than light emitting diodes (LEDs) for micro-projector applications in mobile devices. Unfortunately, the high coherence of a conventional laser produces speckle noise due to interference of the scattered light from a projection screen. For fluorescence microscopy, visible lasers are often used as excitation sources, because the common fluorescent dyes have absorption bands in the visible spectrum. Even though the fluorescence is spatially incoherent and does not produce speckle, coherent artifacts produced by the pump laser result in spatially non-uniform illumination which then corrupts the fluorescence image. Coherent artifacts can be introduced by aberration of the imaging optics, and interface reflection or edge diffraction. To remove the coherent artifacts, various compounding techniques have been developed, e.g., using diffractive elements [1], or rotating/vibrating components [2][3][4]. Since these methods involve mechanically moving parts, they are relatively slow and require long acquisition times, which limit the application for high-speed imaging or projection.A different approach to prevent speckle formation is developing lasers with low spatial coherence. A careful design of the laser cavity can facilitate lasing in many spatial modes with distinct emission pattern. The total emission from those mutually incoherent lasing modes has low spatial coherence, thus enabling speckle-free fullfield imaging. To date, several types of such lasers have been developed, including dye-based random lasers [5], powder-based random Raman lasers [6], solid-state degenerate lasers [7], semiconductor-based chaotic microcavity lasers [8], broad-area vertical cavity surface emitting lasers (VCSELs) [9][10][11], VCSEL arrays [12], and degenerate vertical external cavity surface emitting lasers (VECSELs) [13]. All of these lasers except the dyebased random lasers [5] and the powder-based random Raman lasers [6] have emission beyond the visible spectrum, making them unsuitable for imaging or display applications that require visible light. The dye-based random lasers and the powder-based random Raman lasers, though emitting in the visible spectrum, have high lasing thr...

Complex lasers with controllable coherence

“…For example, an aspheric or diffractive beam shaper transforms a Gaussian intensity profile of a single-mode laser beam to a flat-top beam. However, a multimode degenerate laser can directly generate an output beam with flat-top profile 93 . In fact, any beam profile can be produced by inserting an amplitude mask inside a degenerate cavity close to the output coupler or the end mirror.…”

Parallel random LiDAR with spatial multiplexing of a many-mode laser

“…Compared to spectral demultiplexing, spatial demultiplexing is able to provide many channels without sacrificing the bandwidth and resolution of the individual channel. Moreover, our source has a intrinsic bandwidth of 100 GHz and can thus generate simultaneously several independent channels in various RF bands of interest for RADAR, such as the K u band (12-18 GHz), the K a band (27)(28)(29)(30)(31)(32)(33)(34)(35)(36)(37)(38)(39)(40) and the W band (75-110 GHz), where the last is not accessible with temporal chaos from semiconductor lasers under optical feedback. Without those trade-offs, our parallel ranging scheme based on a high-power many-mode laser will facilitate high-speed 3D sensing and imaging.…”

“…1(a). It consists of two lenses in a 2f − 2f telescope arrangement between two flat mirrors [37]. The focal length f of both lenses is 100 mm, which yields a cavity length L = 4f = 400 mm.…”

Suppressing meta-holographic artifacts by laser coherence tuning

“…To eliminate coherent artifacts of meta-holograms, we resort to a degenerate cavity laser (DCL) with tunable spatial coherence for illumination. The DCL provides a wide tuning range of spatial coherence with little power loss [33][34][35] . Its fast decoherence enables a short exposure time for high-speed imaging 36 .…”