Uncooled operation of the laser diodes up to -45～85 ℃ is an important requirement, because in low-cost transmitter modules thermoelectric cooler of the laser is too expensive and therefore has to be avoided.
We have theoretically investigated a 1.3 ㎛ InGaAsP/InPFP (Fabry- Perot) PBH-LD(Planar Buried Heterostructure-Laser Diode) for high temperature operation with low threshold current. Based on the rate equations, we proposed the electrical equivalent circuit of PBH-LD with p-n-p-n current blocking layer. The leakage width and the concentration of p-InP blocking layer can be expressed by a resistor. Consequently, the resistor must be large to reduce leakage current through leakage path. The electrical equivalent circuit was simulated with PSpice circuit simulator.
The PBH-LD is fabricated by using the vertical type LPE (Liquid Phase Epitaxy) system. The simulation results were as follows. For 300 ㎛ cavity length at 25 ℃, the threshold current and the output power were respectively 6mA and 33 mW with the leakage width of 0.1㎛. Here, the concentration of p-InP blocking layer was 5×1017 cm-3. In order to reduce the leakage current in the electrical equivalent circuit of PBH-LD, we need to decrease the concentration of p-InP blocking layer because it means the increase of the resistor.
The output power of the fabricated PBH-LD was measured. From the measurement, low threshold current of 6 mA was obtained in 300 ㎛ cavity length PBH-LD at 25 ℃. Also, the output power of 29 mW was obtained at about 100 mA.
From the comparison between simulation and measurement, we confirmed that the threshold current has same value of 6 mA and the output power of the simulation was larger than measurement because the electrical equivalent circuit wasn't been considered with the dependence of temperature.
Hence, if we consider the temperature parameter in the electrical equivalent circuit of PBH-LD, we will achieve more accurate analysis compared with the measurement.
In addition, if we could control the precise growth time and temperature in the fabrication of LD, we may fabricate LDs which operates with very low threshold current, high output power, and stable transverse mode at high temperature up to 85 ℃.