Due to the birefringence effect of the quantum well laser and the collimating lens in a low temperature cryostat, the polarization of the output light was changed, therefore the photon number fluctuation of the output individual linearly polarized mode was increased. By compensating the birefringence with a quarter wave plate, and utilizing an inphase external weak feedback to laser diode laser by means of residuary reflection from the two surfaces of a half wave plate, we obtained the output light of 24mW with the photon number fluctuation 2.8dB below the Standard Quantum Limit at the pump current of 34.65mA.