A terahertz (THz) photo-mixing with a THz wave photo-mixer module using a uni-traveling-carrier photodiode (UTCPD) and home-built 1 μm-band ASE-free tunable external-cavity diode lasers (ECDLs) provides a narrow-band (40 MHz) wide range (up to 4.5 THz) coherent tunable THz light source system. Obtained THz-waves reach 100 nW at 0.9 THz and 100 pW at 4.0 THz. The difference frequency between mixing lights can be tuned over 20 THz, and the frequency tuning has a resettability and an accuracy corresponding to the estimation error of FSR 270 MHz hollow-core etalon as a frequency calibrator, around 1 MHz/THz. Some of dips in the frequency dependence of THz-waves caused by water vaper absorption reach a noise floor of this system, so the dynamic range of this system is demonstrated at least 40 dB in power ratio.
In order to overcome catastrophic optical damage, decoupled confinement heterostructure (DCH) featuring a broadened waveguide and thin carrier block layers have been developed. Due to decoupling of carrier and optical confinement, a DCH laser can be designed more flexibly than a conventional separated confinement heterostructure laser, i.e., laser diodes can be designed with a variety of gain coupling factor (Gamma) (perpendicular), quantum-well number NW, keeping the beam divergence angle constant.
High power InGaAs/AlGaAs laser diodes with decoupled confinement heterostructure (DCH) have been developed. Almost Al-free waveguide and cladding layers were realized in 980 nm DCH laser diodes without degrading temperature characteristics. The extremely low electrical and thermal resistances allowed high power and efficient operation. The maximum CW output power as high as 9.5 W was obtained with 100-micrometer-aperture broad area. DCH laser diode. The maximum efficiency was 55% at 2.5 W. The series resistance of 1.8-mm long cavity was 0.04(Omega) and internal loss was 1.5 cm-1. The characteristic temperature (T0) was 155 K. The substantially Al-free DCH structure enables easy fabrication of various index guided laser diodes. We have developed two types of real index guided laser diodes, buried- ridge and self-aligned structure. Buried-ridge laser diode presented 1.3 W maximum CW output power and 500 mW single mode operation. Self-aligned structure laser diodes showed 1.4 W CW output power and 700 mW single mode operation with better reproducibility.
High Power GaAs/AlGaAs laser diodes with a decoupled confinement heterostructure (DCH) have been developed. This novel structure features broadened waveguide layers and thin carrier block layers sandwiching an active layer. Catastrophic optical damage (COD) level was twice as high as the corresponding separated confinement heterostructure (SCH) laser diode due to the improvement of mode profiles. Al- content of cladding layers is greatly reduced in DCH laser diode without degrading temperature characteristics. The decrease of electrical and thermal resistivities allows high- power and high-efficiency operation. CW output, 4.6 W was obtained with a 50 micrometer-aperture 809 nm DCH laser diode. The maximum efficiency was 49% at 2.8 W. Life test was carried out over 2,000 hours under the conditions of 1.0 W - 50 degrees Celsius. The median life was estimated to be more then ten thousand hours at this condition. Decoupled confinement heterostructure is advantageous for the fabrication of the index guided structure, since the reduction of chemically active Al-composition relieves the process difficulties related to the chemical etching and the selective re-growth. Index guided laser diode with a buried ridge structure presented 400 mW single mode operation at 860 nm. The life test was carried out under the conditions of 300 mW - 50 degrees Celsius. All the 25 devices showed no failure up to 7,000 hours.
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