Photonic-crystal Distributed-feedback Lasers Research Articles

Photonic crystal distributed feedback quantum cascade laser fabricated with holographic technique

Electrically pumped, edge-emitting, singlemode operation of a two-dimensional photonic crystal distributed feedback (PCDFB) quantum cascade laser emitting at ∼7.8 µm is demonstrated. The two-beam holographic technique combined with wet-etching process is successfully used to define a square-lattice PCDFB structure on the top grating layer of the laser. This simple PC fabrication method may open exciting opportunities for the wide application of PCDFB lasers.

Low threshold two-dimensional organic photonic crystal distributed feedback laser with hexagonal symmetry based on SiN

We report on the characterization of low threshold second-order organic photonic crystal distributed feedback laser based on SiN with hexagonal lattice. SiN can increase the index contrast in the photonic crystal distributed feedback as well as the confinement in the waveguide which results in a decrease in the threshold. Low threshold energy of 0.65kW∕cm2 and full width at half maximum of 0.9nm were achieved in lasers. As a result, we observe surface-emitting lasing action and hexagonal emission pattern of side beams whose direction slightly deviates from the normal. We explain the feedback mechanisms by using semiclassical laser theory.

Broad-stripe, single-mode, mid-IR interband cascade laser with photonic-crystal distributed-feedback grating

We have demonstrated an electrically pumped photonic-crystal distributed-feedback laser with an interband cascade active region emitting at 3.3μm. At 78K, the stripe of width 400μm emits up to 67mW of cw power into a single spectral mode with side-mode suppression ratio ≈27dB. The full width at half maximum of the far-field divergence angle is ≈0.5°, which combined with the near-field profile yields an effective M2 of 1.7–2.0.

Electrically Pumped Photonic Crystal Distributed Feedback Quantum Cascade Lasers

ABSTRACTIn parallel with the effort to improve the efficiency of Quantum cascade lasers (QCL) for high power continuous wave (CW) operations, the peak power in pulsed mode operation can be easily scaled up with larger emitting volumes, i.e., processing QCLs into broad area lasers. However, as the emitter width increases, multi-mode operation happens due to poorer lateral mode distinguishability. By putting a two dimensional photonic crystal distributed feedback (PCDFB) layer evanescently coupled to the main optical mode, both longitudinal and lateral beam coherence can be greatly enhanced, which makes single mode operation possible for broad area devices. For PCDFB laser performance, the linewidth enhancement factor (LEF) plays an important role in controlling the optical coherence. Being intersubband devices, QCLs have an intrinsically small LEF, thus serves better candidates over interband lasers for PCDFB applications. We demonstrate electrically pumped, room temperature, single mode operation of photonic crystal distributed feedback quantum cascade lasers emitting at λ ∼ 4.75 μm. Ridge waveguides of 50 μm and 100 μm width were fabricated with both PCDFB and Fabry-Perot feedback mechanisms. The Fabry-Perot device has a broad emitting spectrum and a broad far-field character. The PCDFB devices have primarily a single spectral mode and a diffraction limited far field characteristic with a full angular width at half-maximum of 4.8 degrees and 2.4 degrees for the 50 μm and 100 μm ridge widths, respectively.

Spectral and spatial single mode emission from a photonic crystal distributed feedback laser

The authors report on the fabrication and characterization of photonic crystal distributed feedback lasers, which are a generalization of distributed feedback and angled distributed feedback lasers. Spectral and spatial mode selection is provided by a two-dimensional, rectangular lattice of etched air holes that is tilted relatively to the cleaved facets. InGaAs∕AlGaAs laser layers emitting around 1μm were used for the fabrication of the devices. The laser emits at a single wavelength with a side mode suppression ratio of 50dB. The emission is concentrated in a narrow far field, indicating that a coherent mode is formed over the entire width of the device.

Surface-emitting photonic-crystal distributed-feedback laser for the midinfrared

We demonstrate a surface-emitting photonic-crystal distributed-feedback laser, emitting at λ≈3.7μm from an antimonide type-II “W” active region. While single-mode operation was not achieved, the spectral emission linewidth for cw optical pumping was only 1.3Å. For pulsed operation with a pump-spot diameter of 0.8mm, 60% of the nearly circular output beam was transmitted through an aperture with an angular diameter approximately five times that of a Gaussian emitted beam, assuming that the emission area and pump area are the same.

Broad-stripe midinfrared photonic-crystal distributed-feedback lasers with laser-ablation confinement

We investigate broad-stripe photonic-crystal distributed-feedback lasers emitting at λ=4.4 μm. The beam quality improves dramatically when a laser ablation technique is used to eliminate parasitic Fabry–Pérot-like modes, by inducing loss in the regions adjoining the pump stripe. Even though the peak of the gain spectrum never fully aligns with the grating resonance, for pulsed pumping the output beam is essentially diffraction limited up to a stripe width of 150 μm, and remains no worse than six times the diffraction limit for stripes as broad as 400 μm.

Design optimization for high-brightness surface-emitting photonic-crystal distributed-feedback lasers

A new time-domain Fourier-Galerkin (TDFG) theory is developed to simulate the near-field, far-field and spectral characteristics of surface-emitting photonic-crystal distributed-feedback (SE PCDFB) lasers. It is found that a properly-designed two-dimensional hexagonal or square-lattice grating should efficiently couple the output into a single SE mode that retains coherence for aperture diameters of up to /spl ap/1 mm. We identify lattice structures and precise conditions under which all components of the transverse electric or transverse magnetic polarized optical fields constructively interfere to produce a single-lobed, near-diffraction-limited circular output beam. The TDFG simulations predict that quantum efficiencies as high as 30% (60% if reflectors are built into the waveguide structure) should be attainable. A surprising conclusion is that diffractive coupling into the surface-emitting direction must be relatively weak, in order to assure selection of the desired symmetric in-phase mode. Furthermore, gain media with a moderate linewidth enhancement factor should produce the best SE PCDFB performance, whereas edge emitters nearly always benefit from a very small value.

Photonic-crystal distributed-feedback lasers for the midwave infrared

Theoretical modelling indicates that photonic-crystal distributed-feedback (PCDFB) lasers should be capable of providing near-diffraction-limited, spectrally pure sources of midwave-infrared radiation. The authors have studied both 2nd-order and 1st-order optically pumped PCDFB lasers with ‘W’ active regions. Whereas it was not possible to optimise the 2nd-order grating due to the limitations of optical lithography, the 1st-order grating was fabricated using electron-beam lithography with a nearly optimum set of coupling coefficients. The resonant wavelength of the 1st-order grating remained somewhat longer than the gain peak at all temperatures up to the maximum for laser operation. In spite of the substantial detuning, the beam quality improved by a factor of five over angled-grating (α‐DFB) lasers fabricated from similar material, and was slightly better than that for the 2nd-order grating at stripe widths <175 µm. Although the devices were operated in the pulsed mode, a considerable narrowing of the multi-mode spectral linewidth was also realised.

High-brightness mid-infrared photonic-crystal distributed-feedback lasers

The far-field emission and spectral characteristics of a photonic-crystal distributed-feedback (PCDFB) laser with an antimonide type-II ‘W’ active region were studied theoretically and experimentally. A 2nd-order grating was defined on a 2D rectangular lattice tilted by 20° relative to the facet normal, using optical lithography and dry etching. For pulsed optical pumping, the emission line centred on λ = 4.6–4.7 μm was up to an order of magnitude narrower (7–10 nm) than those of Fabry-Pérot and angled-grating DFB (α-DFB) lasers fabricated from the same wafer. The PCDFB beam quality was also substantially enhanced, for example, by a factor of 5 compared to the α-DFB at a pump-stripe width of 200 μm. Theoretical simulations based on a self-consistent time-domain Fourier-transform simulation confirm significant advantages of the PCDFB laser over an α-DFB geometry.

Mid-infrared photonic-crystal distributed-feedback lasers

Photonic-crystal distributed-feedback (PCDFB) lasers have the potential to provide near-diffraction-limited, spectrally pure sources of midwave-infrared radiation. For a first proof-of-principle demonstration, optical lithography and dry etching were used to pattern a second-order two-dimensional rectangular lattice whose grating was tilted by 20° relative to the facet normal. The antimonide type-II “W” active region emitted at λ=4.6–4.7 μm. For pulsed optical pumping, the emission line was much narrower (7–10 nm) than those of Fabry–Perot and angled-grating DFB ( α-DFB) lasers fabricated from the same wafer, and the beam quality was enhanced by as much as a factor of 5 compared with the α-DFB. The observation of two distinct lines in the PCDFB spectrum is attributed to a near-degeneracy of grating resonances at two different symmetry points of the Brillouin zone for the rectangular lattice. Quantum-cascade PCDFB lasers with line width enhancement factors close to zero are projected to emit a single-mode output even when the gain stripe is as wide as 1 mm.

Mid-infrared photonic-crystal distributed-feedback laser with enhanced spectral purity and beam quality

We report a photonic-crystal distributed-feedback (PCDFB) laser with an antimonide type-II “W” active region. Optical lithography and dry etching were used to pattern the two-dimensional rectangular lattice with a second-order grating tilted by 20° relative to the facet normal. For pulsed optical pumping, the emission line centered on λ=4.6–4.7 μm is considerably narrower (7–10 nm) than for Fabry-Pérot and angled-grating DFB (α-DFB) lasers fabricated from the same wafer. The PCDFB beam quality is also substantially enhanced, e.g., by a factor of 5 compared with the α-DFB at a pump-stripe width of 200 μm.

Photonic-crystal distributed-feedback lasers

The far-field emission characteristics and spectral purity of photonic-crystal distributed-feedback (PCDFB) lasers, in which the grating is defined on a two-dimensional lattice that is tilted with respect to the facets, are analyzed using a self-consistent time-domain simulation. It is shown that both conventional DFB and angled-grating DFB devices are special cases of the PCDFB laser, which combines and enhances the best features of each. The simulations project both high beam quality and high spectral purity out to pump stripes of width ≫100 μm.