Laser diodes in photon number squeezed state

A laser diode with an intrinsic layer as the space charge limited current region is expected to emit a low noise (less than the shot noise level) light. However, when one applies the intrinsic layer to the laser diode, severe difficulty is faced. Because the intrinsic layer has a very high resistivity, the applied voltage to operate the laser diode is too large and causes catastrophic damage to the laser diode. Here we propose novel laser diodes which emit a low noise light. The first is an AlGaAs laser diode having an undoped layer between the active layer and the cladding layer which acts as the space charge limited current region. Fano factor, F/sub m/, of this laser diode is 28% smaller than the shot noise level (standard quantum limit, F/sub m/=1) at 21 mA (output power, P/sub 0/=20 mW). The second one is an InGaAsP laser diode having two tunnel barrier layers whose bandgap energy is larger than that of the cladding layer. The region between the barriers acts as the space charge limited current region, Fano factor, F/sub m/ of this laser diode is 47% smaller than the shot noise level at 21 mA (P/sub 0/=10 mW). On the other hand, an AlGaAs laser diode with the two tunnel barrier layers has Fano factor, F/sub m/ which is 43% smaller than the shot noise level at 21 mA (P/sub 0/=20 mW). The calculated amplitude noise spectral densities of the latter two laser diodes are in good agreement with the calculated values from Langevin method. However, the calculated amplitude noise spectral density of the former laser diode does not agree with the calculated value from Langevin method. This disagreement is also discussed.