Phase-modulating lasers toward on-chip integration

Multi-focusing surface-emitting lasers

Complete control of a beam pattern requires not only projecting a two-dimensional (2D) pattern but also focusing on a three-dimensional (3D) point cloud, which is typically achieved utilizing holography under the framework of diffraction. We previously reported direct focusing from on-chip size surface-emitting lasers that utilize a holographically modulated photonic crystal cavity based on 3D holography. However, this demonstration was of the simplest 3D hologram with a single point and single focal length, and the more typical 3D hologram with multiple points and multiple focal lengths has not yet been examined. Toward direct generation of a 3D hologram from the on-chip size surface-emitting laser, we here examined a simple 3D hologram featuring two different focal lengths with a single off-axis point in each to reveal the fundamental physics. Two types of holography, one based on superimposition and the other on random tiling, successfully demonstrated the desired focusing profiles. However, both types caused a spot noise beam in the far field plane due to interference between focusing beams with different focal lengths, especially in the case of the superimposing method. We also found that the 3D hologram based on the superimposing method consisted of higher order beams including the original hologram due to the manner of the holography. Secondly, we demonstrated a typical 3D hologram with multiple points and focal lengths and successfully showed the desired focusing profiles by both methods. We believe our findings will bring innovation to mobile optical systems and pave the way to developing compact optical systems in areas such as material processing, micro fluidics, optical tweezers, and endoscopy.

On-chip size, low-noise fringe pattern projector offering highly accurate 3D measurement

Fringe pattern projectors are quite useful for highly accurate three-dimensional (3D) measurement when a projector or LED array is used for illumination. We have fabricated a 0.2 mm × 0.2 mm structured light source, which was an on-chip size surface-emitting laser that utilized a holographically modulated two-dimensional (2D) photonic crystal (PC). This will make possible an extremely compact 3D measurement system that will positively impact mobile systems. However, the fringe pattern tends to cause speckle-like noise that leads to severe positional error in 3D measurement. Here we present a simple approach to projecting a low-noise fringe pattern from our surface-emitting lasers by using a one-dimensional (1D) focusing hologram. This method improves the flatness of the fringe pattern by around four times.

Direct-focusing surface-emitting laser

Focusing is a fundamental optical technique that has been widely implemented via lenses. Here, we demonstrate direct focusing from a band-edge surface-emitting laser, whose emission area is 200 µm × 200 µm, without any lenses. To achieve this, a phase-modulating layer is incorporated into the laser cavity. This layer acts simultaneously as a lasing cavity similar to that of a photonic crystal laser and as a holographic spatial-phase modulator, which transforms the output beam into a focusing beam by slightly shifting the positions of holes from a periodic square lattice. Beam profiles along the surface normal clearly show that direct focusing occurs with a focal length and focal spot size of 310 µm and 6.1 µm, respectively. The focal length agrees well with the theoretical value, and the focal spot size is 2.0 times the diffraction-limited size, which indicates that the higher transverse modes are sufficiently suppressed. In addition, the power density at the focus is 540 times higher than that at the near-field plane. Interestingly, a focus pattern is also observed in the opposite direction at the near-field plane, which indicates that a converging beam and a diverging beam are simultaneously emitted because of the nature of the in-plane band-edge laser. The conventional beam patterns of semiconductor laser cavities are limited to the regime of two-dimensional projection based on a Fourier hologram. In contrast, we demonstrate the simplest form of a three-dimensional point cloud based on a Fresnel hologram, which is quite useful for micro-sensing applications such as microfluidics, flow cytometry, blood sensors, and endoscopy.

Vertically integrated diffractive gratings on photonic crystal surface emitting lasers

We designed and fabricated a photonic crystal surface emitting laser (PCSEL) with vertically integrated diffractive optical elements on their top to study the mechanism of static beam steering on a single chip. The deflected output beam by the self-formed periodic ITO cladding layer of the PCSEL can be further steered by changing the grating period and azimuthal angle of the diffractive gratings relative to the photonic crystal. Through the analysis of photonic band structure and lasing characteristics, the periodic ITO structure is coupled to the photonic crystal band, whereas the integrated grating serves the diffractive function only. The findings pave the way for the design of PCSELs enabling single or multiple output beam with varying direction capability. This type of laser is regarded as an ideal light source for various applications, such as light detection and ranging and three-dimensional sensing systems.

200×200 µm2 structured light source

3D structured illumination is important in high-speed 3D metrology where beam patterns are roughly categorized into multi-dot and fringe patterns. For example, large-scale multi-dot patterns are utilized for facial recognition in an iPhone X on the basis of an active stereo method, while fringe patterns are utilized in Grey code patterns or fringe projection profilometry including Fourier transform profilometry and the phase shifting profilometry, which is suitable for high-resolution measurement. Among these applications, the light sources include a combination of vertical-cavity surface-emitting lasers (VCSELs) and diffractive optical elements (DOEs), a projector, and so on. Recently, we demonstrated static arbitrary two-dimensional beam patterns without a zero-order beam from needle-tip sized integrable spatial-phase-modulating surface-emitting lasers (iPMSELs). Due to their compactness (they are one order of magnitude smaller than DOE), surface-emitting device, lack of zero-order beam, and ease of switching the beam patterns electrically, iPMSELs will be suitable as an ultra-compact light source for 3D metrology that not only downsizes the conventional light source but also contributes to 3D inspections in narrow spaces such as dental and endoscope examinations. In this context, we have examined two beam patterns (multi-dot and fringe) both without a zero-order beam by using the iPMSELs. In the former, we have demonstrated projection of large-scale dot patterns of more than 10,000 points, which is the same order of magnitude as points in a practical device from a 200×200-µm² emitter. Since the emitter has approximately 1 mega scattering points, this structure enables 1-mega-pixel images in the wavenumber space, which are comparable to the images of a typical projector emitting several-mega-pixel images from several tens of centimeters. In the latter, we successfully shifted the fringe patterns, which is vital to applying the phase shifting profilometry, despite the superposition of the conjugate ±1st order beam patterns.

Removal of surface-normal spot beam from on-chip 2D beam pattern projecting lasers

Static arbitrary two-dimensional beam patterns have been demonstrated using on-chip size "integrable spatial-phase-modulating surface-emitting lasers," which use the band edge mode of a two-dimensional photonic crystal as an in-plane resonator, while the spatial phase of the lightwaves of the band edge mode are simultaneously modulated in a holographic manner by a local positional shift of holes from their lattice points. Meanwhile, the beam patterns include a spot beam in the surface-normal direction (0th-order beam), which corresponds to components of vertical diffraction of the band edge modes without spatial phase modulation. A promising method, used to remove the 0th-order beam, uses a structure that prohibits the vertical diffraction of band edge modes. For this purpose, we set the period of the virtual photonic crystal structure from the conventional Γ₂ band edge to the Μ₁ band edge, where vertical diffraction is prohibited. Moreover, the additional spatial phase modulation that cancels the in-plane component of the wavevectors of the lightwaves of the band edge modes at the Μ₁ band edge are also imposed to output the beam patterns vertically. As a result, we successfully demonstrated two-dimensional beam patterns without a spot beam in the surface-normal direction.

Principle of beam generation in on-chip 2D beam pattern projecting lasers

Integrable spatial-phase-modulating surface-emitting lasers, which utilize the band edge mode of two-dimensional photonic-crystals as resonators, project static arbitrary two-dimensional beam patterns from on-chip size. In this device, holes shifting from the lattice point of a two-dimensional photonic crystal provide spatial phase modulation to light waves, which form standing waves in the resonator. Thus far, the origin of the beam patterns has not been studied, especially the formation of subsidiary beam patterns against the designed beam pattern. In this work, we clarify the origin of beam patterns in two types of spatial phase modulating method, which impose in-plane shifting of holes according to circular and linear shift methods. Based on a theoretical study of spatial phase modulation, we reveal that the circular shift method provides a symmetric beam pattern, while the linear shift method causes an asymmetric beam pattern. Consequently, we demonstrated the asymmetric two-dimensional beam pattern by the linear shift method for the first time.

Design of photonic-crystal surface-emitting lasers with circularly-polarized beam

We propose new structure of photonic-crystal surface-emitting lasers with oblique-triangular-prism-shaped air holes for direct emission of circularly polarized beam. We show appropriate height and tilt angle of oblique-triangular-prism-shaped air holes to achieve high degree of polarization. Secondly, we investigate the influence of cavity length. High degree of polarization can be obtained by appropriate air-hole shape and cavity length. We also show that right-handed or left-handed circular polarization can be chosen by changing tilt direction of air holes.