Mothi Madhan Raj

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.

Multiple Micro-Cavity Laser with Benzocyclobutene/Semiconductor High Reflective Mirrors Fabricated by CH 4/H 2-Reactive Ion Etching

Multiple Micro-Cavity (MMC) lasers consisting of narrow and deep grooves buried with a Benzocyclobutene (BCB) polymer, were realized by the CH4/H2-reactive ion etching (RIE) process. A threshold current of 30 mA was obtained at 200 K for the micro-cavity length lH = 5.1 µm (groove width lL = 183 nm, pitch \mathnormalΛ = 5.3 µm, total cavity length L = 300 µm, stripe width Ws = 5 µm) while also showing a stable single-wavelength operation. Room temperature operation of an MMC laser consisting of 3λ/4-BCB buried grooves (= 0.70 µm) was also obtained with a threshold current as low as 18 mA for the total cavity length of 200 µm (\mathnormalΛ = 20 µm, 10 elements, Ws = 5 µm), and the effective reflectivity of the MMC structure was estimated to be 94%.

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.

GaInAsP/InP Multiple Short Cavity Laser with λ/4-Air Gap/Semiconductor Bragg Reflectors

A room temperature operation of multiple cavity (MC) laser consisting of λ/4-air gap (=0.39 µm) and semiconductor reflectors was fabricated using two steps organometallic vapour-phase epitaxy (OMVPE) and two steps wet chemical etching. The threshold current as low as 78 mA (Jth = 1.25 kA/cm2) was obtained for a total cavity length 164 µm (Pitch Λ: 20 µm; 8 elements) and stripe width of 40 µm. The effective power reflectivity of this cavity structure was estimated to be higher than 92%.

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.

Photon Recycling Effect in Semiconductor Lasers using Low Dimensional Structures

The photon recycling effect in quantum film, quantum wire, and quantum box has been theoretically investigated using rate equation analysis and the density-matrix method, to obtain further reduction in the threshold current. The threshold reductions due to photon recycling in quantum film, wire, and box, in the case of lattice matched Ga0.47In0.53As/InP, are 40%, 24%, and 0%, respectively, for a fixed cavity loss of lasing mode normalized by the optical confinement factor α L/ξ L=50 cm-1. The estimation also shows that photon recycling is more effective in compressively-strained (CS) Ga0.18In0.82As0.73P0.27/InP quantum structures than in lattice-matched quantum structures. In both cases, the threshold reduction due to photon recycling is larger in the quantum film structure than in quantum wire and box structures, because the deviation between the peaks of the gain and the spontaneous emission spectra is large in the quantum film structure.

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.

High reflectivity laser facets by deeply etched DBR buried with benzocyclobutene

Lasers having a DBR facet on one side of the cavity were fabricated in a simple process using one step electron beam (EB) lithography and CH/sub 4//H/sub 2/-reactive ion etching (RIE). The DBR structure consists of deeply etched 3/spl lambda//4 wide grooves, which are buried with benzocyclobutene (BCB) polymer so as to reduce diffraction loss, spaced 3h/4 apart. The reflectivity of the DBR was estimated from the threshold current dependence on cavity length and an output power ratio from the front to the rear facets to be as high as 96% for 10 element DBR.