We demonstrated direct surface-emitting of Laguerre–Gaussian beams with wavefront modulated lasers. This integrable phase-modulating surface-emitting lasers has potential to emit arbitrarily configured beam patterns without requiring any optical elements or scanning devices. The fabricated devices are on-chip-sized, making them suitable for integration. We introduce a phase-modulating resonator in a semiconductor laser, which analogically behaves as phaseonly holograms, kinoform, to allow the concurrent realization of lasing and phase modulation. Particularly, this is promising in the use for free-space optical communications due to the fact that coaxial propagation of orbital angular momentum (OAM) properties with different OAM mode states are mutually orthogonal.
Recently, W-class photonic-crystal surface-emitting lasers (PCSELs) with both a single spectrum and narrow spot beam pattern are reported. These highly coherent PCSEL properties cause a highly bright laser light that is useful for various applications. To improve the PCSEL output power, it is important to enlarge the emitting area to reduce the heat generation effect. However, multi-mode oscillation occurs in a broad emitting area because the difference in the threshold gain between the fundamental and higher modes becomes narrower as the emitting area is broadened. In this work, we fabricate PCSELs with double-hole lattice points that decrease the optical confinement to prevent multi-mode oscillation. The fabricated device, consisting of an AlGaAs/InGaAs material system designed to be oscillated at a wavelength of 940nm, has an emitting area of 300 × 300 μm<sup>2</sup>. In a square lattice photonic crystal whose lattice period equals the lasing wavelength embedded in PCSELs, the distance between the centers of the double hole is set to one quarter of the lasing wavelength to decrease in-plane coupling caused by interference. We confirm that this device is oscillated at the Γ point of band edge A in the photonic band structure. The peak power is more than 5 W under pulse operation at 10 A. The device has a narrow beam divergence of less than 1° and single lobe spectrum in spite of the broad emitting area, so these double-hole lattice points are an effective structure to improve the PCSEL output power.
Photonic crystal surface emitting lasers (PCSELs) have recently been achieved with both a single spectrum and narrow spot beam pattern under several hundred mW of output power. Even though the high coherence properties of PCSELs are expected to be used for various applications, we have focused on a pumping light source for a wavelength conversion system in this work. We fabricated a 1.06 μm PCSEL with a square lattice 2D photonic crystal in which the lattice period corresponded to the lasing wavelength to obtain green light. The fabricated device had a narrow spot beam pattern of less than 0.5 degrees and a single spectrum at 1068 nm under CW output power of more than 200 mW despite the broad emitting area of 200 × 200 μm2. The wavelength conversion system used single pass second-harmonic generation (SHG) that consisted of only the PCSEL and 50 mm long bulk MgO doped periodically with poled lithium niobate (MgO:PPLN) as a nonlinear medium, i.e., it was a lens-free system. It was important to maintain the high brightness of the pumping light in this system with a single spectrum through the MgO:PPLN. As a result, SHG light was obtained at 534 nm with a narrow spot beam pattern, which followed the beam quality of the PCSEL under CW operation.
The photonic-crystal surface-emitting laser (PCSEL) is an attractive semiconductor laser in which a thin two-dimensional photonic-crystal (2D-PC) layer is incorporated into the ordinary broad area edge-emitting laser structure to control the longitudinal-transverse mode owing to diffraction. In principle, the zero group velocity effect at the band edge of the 2D-PC is utilized as a resonator and can be used for the unique properties including large-area coherent oscillation as well as arbitrary beam controlling, which includes the polarization, beam patterns, directions, and generation of vector beams. We investigated the PCSEL toward realizing a practical device that has high power and high beam quality. Here, we show our recent progress. The device structure, which consists of an InGaAs/AlGaAs material system on n-GaAs substrates, is based on an ordinary broad area edge-emitting laser structure except it has a thin 2D-PC layer. The 2D-PC layer is placed near the active layer, and both are embedded between the p and n cladding layers. It is fabricated by using EB lithography, dry etching, and regrowth or MOCVD. The square emitting area has side of 200 micrometers, and transverse modes are well controlled in the entire region. The output power is more than 0.75 W with a single wavelength of 966 nm, and the narrow beam divergence is as narrow as 1° under continuous wave (CW) operation at room temperature. The beam quality is superior with an M<sup>2</sup> of 1.1, which is almost the same as that of the ideal Gaussian beam.