Abstract

Laser diodes with pump noise suppression are reliable sources for the generation of amplitude-squeezed light. Recently, it was shown that this amplitude squeezing is not always a single mode effect but can result from a cancellation among anticorrelated fluctuations between the main mode and competing side modes.¹ To obtain more details on this correlation effect, we performed spectrally resolved intensity noise measurements with a pump noise suppressed SDL 5410-C (810 nm) diode laser in different configurations: free-running, injection-locked, and with an external Littrow grating feedback. The laser radiation was passed through a high-transmission (65%) spectrometer. Both the position and width of the output slit were variable. Thus the main mode and, additionally, a variable number of side modes could be detected on a photodiode behind the spectrometer. Figure 1 shows the results for the injection-locked laser. If only the main mode is detected, the intensity noise, measured at a noise frequency of 30 MHz, is well above the SQL (31.4 dB). When the slit is opened symmetrically around the main mode, the noise decreases exponentially with the number of additionally detected side modes. Amplitude squeezing of 1.3 dB is observed only if more than 150 modes are transmitted simultaneously, clearly indicating that all the diode laser side modes contribute to the anticorrelation with the main mode. To obtain further information on this correlation, the slit was opened asymmetrically such that the main mode was transmitted with an increasing number of short- or long-wavelength side modes, respectively. Figure 1 shows that the intensity noise decreases moderately to 28.7 dB in the first case, and to a much lower value in the second case (13.1 dB). The measurements thus show, for the first time, that the anticorrelation of the side modes with the main mode is spectrally asymmetric. This asymmetry was also observed for the free-running diode and the diode with the Littrow grating feedback. We assume that this asymmetric correlation results from asymmetric nonlinear gain² in the laser diode, which couples the fluctuations of the individual modes. Our results are in good agreement with a Langevin rate equation model³ for three modes (main mode and one additional mode of shorter and longer wavelength, respectively), including gain inhomogeneities due to self-mode gain saturation¹ and asymmetric cross-mode gain saturation.⁴ Figure 2 shows the intensity noise, calculated for different combinations of simultaneously detected modes at a noise frequency of 30 MHz versus the loss parameter p. This parameter is defined as the ratio of the main mode loss rate by the side mode loss rate, as in Marin et al.,¹ and allows a comparison of the investigated setups in a unified approach. The asymmetric noise properties of the injection-locked laser (Fig. 1) are reproduced for a value of log(1−p) = −2.9. The behavior of the free-running and the external feedback diode laser is reproduced for values of log(1−p) < −3.5 and > −2, respectively. Additionally, our model explains the experimentally observed enhancement of low- frequency noise in the case of amplitude- squeezed light generation with injection-locking.⁵ Figure 3 shows the calculated noise spectra of the different modes and the total laser intensity for a loss parameter of log(1−p) = −2.9. Amplitude squeezing of the total intensity is predicted for frequencies from 10 to 100 MHz, whereas the total intensity noise increases towards lower frequencies to values slightly above the SQL.

© 1998 Optical Society of America