J. Wiedmann

1.5-µm-Wavelength Distributed Feedback Lasers with Deeply Etched First-Order Vertical Grating

1.5-µm-wavelength distributed feedback lasers with a deeply etched first order vertical grating were realized for the first time. It was shown that we can obtain an effective coupling by reducing the stripe width. The sample with the cavity length of 430 µm, 1.8 µm stripe width and 0.2 µm grating depth on each lateral side exhibited a 12.5 mA threshold current, 37% total differential quantum efficiency and 35 dB submode suppression ratio at a bias current of two times the threshold.

Polarization-independent all-optical switching in a nonlinear GaInAsP-InP highmesa waveguide with a vertically etched Bragg reflector

We have theoretically designed and experimentally demonstrated polarization-independent all-optical switching in a nonlinear GaInAsP-InP highmesa distributed feedback (DFB) waveguide. The device, which is composed of a highmesa waveguide stripe and a vertically etched Bragg reflector, can be simply fabricated using one-step electron beam lithography and a reactive ion etching process. The device is suitable for integration with other photonic devices such as semiconductor optical amplifiers and wavelength converters. The structural birefringence of the device has a dependence on the waveguide parameters such as the refractive index and thickness of core and cladding. The structural birefringence was successfully eliminated by adjusting the width of the highmesa waveguide. The nonlinear vertical-groove DFB highmesa waveguide is attractive for a polarization-independent all-optical switch from the viewpoint of a large grating coupling coefficient, as compared with a grating-loaded DFB highmesa waveguide. The polarization dependence of the grating coupling coefficient has also been investigated experimentally. It is possible to obtain the polarization-independent grating coupling coefficient by adjusting the grating depth in the vertical-groove DFB highmesa waveguide, together with structural zero-birefringence of the device. Polarization-independent all-optical thresholding and bistable switching operations have been successfully demonstrated in the nonlinear vertical-groove DFB highmesa waveguide.

High-Reflectivity Semiconductor/Benzocyclobutene Bragg Reflector Mirrors for GaInAsP/InP Lasers

Fabrication techniques of high-reflectivity deeply etched semiconductor/benzocyclobutene (BCB) Bragg reflectors by multiple sequential steps of CH4/H2 reactive ion etching (RIE) and O2 plasma ashing are presented. 1.55-µm-wavelength GaInAsP/InP lasers with such reflectors exhibited low threshold current and high differential quantum efficiency with high uniformity. Threshold current as low as 7.2 mA and differential quantum efficiency as high as 50% from the front cleaved facet were obtained for a laser with a 160-µm-long active region and 15 distributed Bragg reflectors (DBRs) on the rear side. The reflectivity of the 15 DBRs was estimated to be as high as 95% from the threshold current dependence on the active region length. Finally, a preliminary aging test under room temperature CW conditions showed stable operation for more than 5000 h.

GaInAsP/InP Distributed Reflector Lasers Consisting of Deeply Etched Vertical Gratings

Deep-etching technology, which is a very simple fabrication process and requires only one lithography and one etching step without any regrowth, is applied to realize 1.5–1.55 µm wavelength GaInAsP/InP vertical grating (VG) distributed reflector (DR) lasers, which consist of a distributed feedback (DFB) region with the VG and a high-reflectivity distributed Bragg reflector (DBR) region. First, the coupling coefficient of the VG is theoretically estimated and fundamental lasing characteristics of the VG-DR lasers are investigated. Then, the fabrication process of the VG-DR lasers and the experimentally achieved fundamental lasing characteristics are given. A threshold current of 12.4 mA and a differential quantum efficiency of 42% from the front cleaved facet are achieved for a 220-µm-long and 5-µm-wide device. A submode suppression ratio (SMSR) of 33 dB at a bias current of 2.4 times the threshold is obtained.

Deeply Etched Semiconductor/Benzocyclobutene Distributed Bragg Reflector Laser Combined with Multiple Cavities for 1.5-µm-Wavelength Single-Mode Operation

Single-mode GaInAsP lasers at a wavelength of 1.5 µm, fabricated using deep dry-etching technology are presented. In the novel design, the high reflectivity of a fifteen-element distributed Bragg reflector (DBR) is combined with the spectral selectivity of multiple cavities (MC) to achieve single-mode operation with high output efficiency. The etched structure was buried with polymer benzocyclobutene (BCB) in order to reduce the diffraction loss between coupled cavities, to passivate the sidewalls of the waveguide and to protect the structure from damage due to process handling. MC lasers with different numbers of cavities were analyzed by the transfer matrix method (TMM) including groove losses and the results of analysis are compared with experimental results of fabricated devices. For a coupled cavity (CC) laser, which has only two cavities, a threshold current of 11 mA and a submode suppression ratio (SMSR) of 36 dB at a bias current of 1.8 times the threshold were obtained for 5-µm-wide mesa stripe geometry.

Theoretical Analysis of GaInAsP/InP Multiple Micro-Cavity Laser.

Fundamental lasing properties of a new type of laser, which consists of a multiple micro-cavity structure with vertical grooves, was theoretically investigated based on an improved perturbation feedback theory in combination with the transfer matrix method. In order to suppress a diffraction loss between semiconductor micro-cavities, an MMC laser having grooves buried with low loss polymer (Benzocyclobutene: BCB) was proposed. Attainable maximum reflectivity was calculated to be higher than 98% for quarter wavelength wide grooves buried with BCB polymer (refractive index of 1.546, number of the micro-cavity elements of 8), which was higher than that of the semiconductor micro-cavity/air groove periodic structure. As a result, a moderately low threshold current operation (1 mA for a stripe width of 1 µm) can be obtained for the total cavity length of 45 µm with the micro-cavity length of 4.90 µm (9 elements).

Continuous Wave Operation of 1.55 µm GaInAsP/InP Laser with Semiconductor/Benzocyclobutene Distributed Bragg Reflector

The room temperature CW operation of a 1.55 µm wavelength GaInAsP/InP laser with a deeply etched third-order Bragg reflector consisting of a semiconductor/Benzocyclobutene periodic structure was obtained for the first time. A threshold current as low as 13.5 mA and differential quantum efficiency of 28% for a cavity length of 330 µm and a stripe width of 5 µm was obtained with 10 elements of the Bragg reflector on one side.

Highly Uniform 1.5 µm Wavelength Deeply Etched Semiconductor/Benzocyclobutene Distributed Bragg Reflector Lasers

The lasing performance of deeply etched distributed Bragg reflector (DBR) lasers was improved to a threshold current as low as 7.2 mA and a differential quantum efficiency as high as 50% with high uniformity. The reflectivity of the 15-DBR reflector was estimated to be as high as 95% from the threshold current dependence on the active region length. A preliminary test under room-temperature CW operation showed stable operation for a duration in excess of 4000 hours.

Highly uniform 1.55 /spl mu/m wavelength lasers with deeply etched semiconductor/benzocyclobutene DBR

1.55 /spl mu/m wavelength GaInAsP lasers with high reflective deeply etched semiconductor/benzocyclobutene (BCB) DBR showing low threshold current and high differential quantum efficiency were successfully obtained with high uniformity. Threshold current as low as 7.2 mA and differential quantum efficiency as high as 50% from the front cleaved facet were obtained with 160 /spl mu/m-long DBR lasers with 15-DBR reflectors on the rear side. The reflectivity was estimated to be as high as 95% from the measurement of the threshold current dependence on the cavity length. Finally, a preliminary aging test under a room temperature CW condition showed stable operation for duration in excess of 300 hours.

Polarization independent all-optical switching employing nonlinear vertical-groove DFB waveguide

We report that the birefringence of waveguide was completely eliminated in a high mesa waveguide with vertically etched grating. Also, we report first demonstration of polarization-independent all-optical switching employing nonlinear DFB waveguide.