Room-temperature Continuous-wave Condition Research Articles

Photonic Crystal Circular Defect (CirD) Laser

We describe the design of photonic crystal circular defect (CirD) lasers to construct a compact optical module with a wavelength division multiplexing function for the application of inter-chip or intra-chip optical interconnects. Subsequently, we investigated the characteristics of CirD lasers including the quality factor of the cavity, the lasing threshold, and the modulation speed with a three-dimensional finite-difference time-domain method and two-dimensional rate equations. Finally, we demonstrated the single mode lasing and wavelength tuning behaviors of the CirD lasers using optical pumping technology under room-temperature continuous-wave conditions.

Monolithic Integration of Membrane-Based Butt-Jointed Built-in DFB Lasers and p-i-n Photodiodes Bonded on Si Substrate

We demonstrate a monolithic integration of lateral-current-injection (LCI)-type membrane-based distributed-feedback (DFB) lasers and p-i-n-photodiodes (PDs) using a butt-jointed built-in (BJB) structure bonded on a Si substrate using benzocyclobutene. The BJB structure to be integrated to the membrane optical devices was prepared by organometallic vapor-phase epitaxy. A threshold current of 280 μA was obtained under a room-temperature continuous-wave condition by adopting a strongly index-coupled DFB laser with a surface grating structure and a λ/4-shift region. A low dark current of 0.8 nA was obtained with the p-i-n-PD at a bias voltage of –1 V, and its photocurrent property coincided with the light output property of the membrane DFB laser.

Ultralow Operating Energy Electrically Driven Photonic Crystal Lasers

The introduction of the photonic crystal (PhC) wavelength-scale cavity as a laser cavity enables us to obtain both ultralow threshold current and operating energy. These parameters are essential when using the transmitters in chip-to-chip and on-chip interconnections. To improve the device performance, we employ an ultracompact embedded active region that we call a lambda-scale embedded active-region PhC laser or LEAP laser. We have developed an electrically driven LEAP laser, which operates under room-temperature continuous-wave conditions. To fabricate the electrically driven LEAP laser, we used Zn thermal diffusion and Si ion implantation, respectively, for p-type and n-type doping in an undoped InP layer. However, with previous fabricated devices there was a large leakage current through the substrate and the threshold current was 0.39 mA, which is larger than the expected threshold obtained by optical pumping. To reduce the leakage current, we propose using an InAlAs sacrificial laser instead of an InGaAs layer. The leakage current path through the substrate is effectively suppressed, and as a result, the threshold current is reduced to 7.8 μA, which is the lowest threshold current reported for any laser. Furthermore, the LEAP laser operates at up to 95 °C by using an InGaAlAs-based multiple quantum well structure. We also describe the dynamic characteristics of the laser. The LEAP laser exhibits a maximum 3-dB bandwidth of 16.2 GHz and the modulation current efficiency factor is 53.8 GHz/mA0.5 or 1.7 GHz/μA0.5, which is four times that of a vertical cavity surface-emitting laser. The device is directly modulated by a 12.5-Gb/s nonreturn-to-zero signal with a bias voltage of 1.6 V and a bias current of 109 μA, resulting in an energy cost of 14.0 fJ/b. This is the smallest operating energy for any laser. These results indicate that the LEAP laser is highly suitable for use as a transmitter in computercom applications.

Emission characteristics of surface second-order metal grating distributed feedback semiconductor lasers

To obtain high-power semiconductor lasers with stable operation in a single longitudinal mode and improve the characteristics of the output beam, an end-emitting surface second-order metal grating distributed feedback (DFB) laser emitting at around 940 nm is fabricated. The characteristics of the uncoated devices with and without gratings are tested under room temperature continuous-wave conditions without any temperature-control device and compared. The devices with gratings achieve high powers of up to 385 mW/facet and a small lateral far-field angle of 2.7° at 1.5 A, have only 4.13 nm/A wavelength-shift, and 0.09 nm spectral linewidth at 600 mA, and operate in a stable longitudinal mode. Devices without gratings operate in multimode, with a larger lateral far-field angle (7.3°) and spectral linewidth (1.3 nm), although with higher output powers. Because of the integration of second-order metal gratings and their very high coupling capability, the output beam quality is improved greatly, the lasing wavelength is stable and varies slowly with changes in injection current, while the spectrum is narrowed dramatically, and the far-field angles are greatly reduced. This opens the way for the realization of watt-scale power broad-stripe (>100 μm) surface second-order metal grating end and surface-emitting DFB lasers and arrays with single frequency, single mode operation and high output beam quality.

GaInAsP/InP lateral-current-injection distributed feedback laser with a-Si surface grating

We fabricated a novel lateral-current-injection-type distributed feedback (DFB) laser with amorphous-Si (a-Si) surface grating as a step to realize membrane lasers. This laser consists of a thin GaInAsP core layer grown on a semi-insulating InP substrate and a 30-nm-thick a-Si surface layer for DFB grating. Under a room-temperature continuous-wave condition, a low threshold current of 7.0 mA and high efficiency of 43% from the front facet were obtained for a 2.0-μm stripe width and 300-μm cavity length. A small-signal modulation bandwidth of 4.8 GHz was obtained at a bias current of 30 mA.

Reduced Temperature Dependence of Lasing Wavelength in Membrane Buried Heterostructure DFB Lasers With Polymer Cladding Layers

An athermal operation of the lasing wavelength in a membrane buried heterostructure distributed feedback (BH-DFB) laser was demonstrated by adopting a polymer (benzocyclobutene) cladding layer which has a negative temperature coefficient of refractive index. Membrane BH-DFB lasers of core thickness 65 nm were operated under room-temperature continuous-wave conditions. The temperature dependence of the lasing wavelength was measured to be 2.45times10-2 nm/degC, which is about 20%-30% of that for conventional semiconductor lasers

Strongly Index-Coupled Membrane BH-DFB Lasers With Surface Corrugation Grating

We realized strongly index-coupled membrane buried-heterostructure (BH) distributed-feedback (DFB) lasers fabricated by electron-beam lithography, CH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> /H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> reactive-ion etching, and regrowth by organometallic vapor-phase epitaxy, using surface corrugation DFB. First, we fabricated narrow stripe membrane BH-DFB lasers with surface corrugation for stable single-mode operation. Under optically pumped room-temperature continuous-wave (RT-CW) operation, a wide stopband width of 68 nm was observed in spite of the narrow stripe width of 0.6 mum. The corresponding index-coupling coefficient of 2950 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> is over two times larger than that of a flat-surface (conventional) membrane BH-DFB laser with a stripe width of 2.0 mum. In addition, we fabricated a short-cavity membrane DFB laser with a surface corrugation structure. A threshold optical pump power of as low as 0.34 mW was realized for a 2.0-mum-wide and 80-mum-long device under RT-CW conditions.

Low-threshold-current operation of 1540 nm GaInAsP∕InP distributed-feedback lasers with multiple-quantum-wire active regions

1540 nm wavelength GaInAsP∕InP distributed-feedback lasers consisting of multiple-quantum-wire active regions with the wire width of 24 nm were realized by electron-beam lithography, CH4∕H2 reactive ion etching, and two-step organometallic vapor-phase-epitaxial growth processes. A threshold current as low as 2.7 mA (threshold current density=270A∕cm2), a differential quantum efficiency of 19%∕facet and a submode suppression ratio of 51 dB at a bias current of twice the threshold were achieved for the stripe width of 3.0μm and the cavity length of 330μm under a room-temperature continuous-wave condition.

Low-threshold distributed reflector laser consisting of wide and narrow wirelike active regions

We demonstrate a low-threshold distributed reflector laser, consisting of a distributed feedback section with wirelike active regions and a passive distributed Bragg reflector section with narrower ones, by utilizing the lateral quantum confinement effect. A threshold current under the room-temperature continuous-wave condition as low as 3.2 mA, which corresponds to a threshold current density of 290 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , was obtained with the stripe width of 4.3 μm and the active region length of 260 μm. The differential quantum efficiency of 19% from the front facet, a high output ratio of 11 from the front facet to the rear facet, and a submode suppression ratio of 47 dB at a bias current 3.75 times the threshold were also achieved.

Low-Threshold Membrane BH-DFB Laser Arrays With Precisely Controlled Wavelength Over a Wide Range

We report on the low-threshold operation of membrane buried heterostructure distributed feedback laser arrays. 45 devices were fabricated on a wafer covering a wide wavelength range of 75 nm. The lowest threshold pump power as low as 0.64 mW along with the submode suppression-ratio of over 30 dB at two times the threshold were successfully obtained at room-temperature continuous-wave condition.

Distributed Reflector Laser with Wirelike Active Regions for Asymmetric Output Property

A distributed reflector (DR) laser with two different wirelike active region widths was realized for the first time. As a result, a threshold current as low as 1.8 mA, an output power ratio of the front to the rear facet of 8 and a submode suppression ratio of 33 dB at 1.2 times the threshold were obtained under a room-temperature continuous-wave condition.

Study of the Effects of Mg and Be Co-Doping in GaN Layers

GaN:Mg/AlGaN single heterostructures were grown on Si(111) substrates by plasma-assisted molecular beam epitaxy (PAMBE) leading to the fabrication of light emitting diodes (LEDs). p-type is successfully achieved using Mg, obtaining hole concentrations in the range of 1 × 1017 cm—3. Although an intense and narrow (8 nm full width at half maximum) ultraviolet electroluminescence is observed under room temperature continuous-wave conditions, the electrical characteristics of the diode suffered mainly from a low p-type doping. In order to obtain higher hole concentrations and increase the performance of these devices, a study of the effects of Mg and Be co-doping is performed. Preliminary results show a decrease of the intensity of the yellow band in the photoluminescence spectra of co-doped samples in comparison with the Be-doped ones together with an increase of the intensity of the donor–acceptor pair related to the Be shallow acceptor. Both effects can be related to an increase in the p-type Be doping efficiency.