Syamsundar De

Intensity noise correlations in a two-frequency VECSEL.

We present an experimental and theoretical study of the intensity noise correlation between the two orthogonally polarized modes in a dual frequency Vertical External Cavity Surface Emitting Laser (VECSEL). The dependence of the noise correlation spectra on the non-linear coupling between the two orthogonally polarized modes is put into evidence. Our results show that for small coupling the noise correlation amplitude and phase spectra remain nearly flat (around -6 dB and 0° respectively) within the frequency range of our interest (from 100 kHz to 100 MHz). But for higher values of the coupling constant the low frequency behaviors (below 1-2 MHz) of the correlation amplitude and phase spectra are drastically changed, whereas above this cut-off frequency (1-2 MHz) the correlation spectra are almost independent of coupling strength. The theoretical model is based on the assumptions that the only source of noise in the frequency range of our interest for the two modes are pump noises, which are white noises of equal amplitude but partially correlated.

Phase Noise of the Radio Frequency (RF) Beatnote Generated by a Dual-Frequency VECSEL

We analyze, both theoretically and experimentally, the phase noise of the radio frequency (RF) beatnote generated by optical mixing of two orthogonally polarized modes in an optically pumped dual-frequency vertical external cavity surface emitting laser (VECSEL). The characteristics of the RF phase noise within the frequency range of 10 kHz-50 MHz are investigated for three different nonlinear coupling strengths between the two lasing modes. In the theoretical model, we consider two different physical mechanisms responsible for the RF phase noise. In the low frequency domain (typically below 500 kHz), the dominant contribution to the RF phase noise is shown to come from the thermal fluctuations of the semiconductor active medium induced by pump intensity fluctuations. However, in the higher frequency domain (typically above 500 kHz), the main source of RF phase noise is shown to be the pump intensity fluctuations which are transferred to the intensity noises of the two lasing modes and then to the phase noise via the large Henry factor of the semiconductor gain medium. For this latter mechanism, the nonlinear coupling strength between the two lasing modes is shown to play an important role in the value of the RF phase noise. All experimental results are shown to be in good agreement with theory.

Experimental demonstration of a dual-frequency laser free from antiphase noise.

A reduction of more than 20 dB of the intensity noise at the antiphase relaxation oscillation frequency is experimentally demonstrated in a two-polarization dual-frequency solid-state laser without any optical or electronic feedback loop. Such behavior is inherently obtained by aligning the two orthogonally polarized oscillating modes with the crystallographic axes of a (100)-cut neodymium-doped yttrium aluminum garnet active medium. The antiphase noise level is shown to increase as soon as one departs from this peculiar configuration, evidencing the predominant role of the nonlinear coupling constant. This experimental demonstration opens new perspectives on the design and realization of extremely low-noise dual-frequency solid-state lasers.

Class-A dual-frequency VECSEL at telecom wavelength

We report class-A dual-frequency oscillation at 1.55 μm in a vertical external cavity surface emitting laser with more than 100 mW optical power. The two orthogonal linear polarizations of different frequencies oscillate simultaneously as their nonlinear coupling is reduced below unity by spatially separating them inside the active medium. The spectral behavior of the radio frequency beatnote obtained by optically mixing two polarizations and the phase noise of the beatnote have been explored for different coupling strengths between the lasing modes.

Intensity- and phase-noise correlations in a dual-frequency vertical-external-cavity surface-emitting laser operating at telecom wavelength

The amplitude and phase noises of a dual-frequency vertical-external-cavity surface-emitting laser (DF-VECSEL) operating at telecom wavelength are theoretically and experimentally investigated in detail. In particular, the spectral behavior of the correlation between the intensity noises of the two modes of the DF-VECSEL is measured. Moreover, the correlation between the phase noise of the radio-frequency beat note generated by optical mixing of the two laser modes with the intensity noises of the two modes is investigated. All these spectral behaviors of noise correlations are analyzed for two different values of the nonlinear coupling between the laser modes. We find that to describe the spectral behavior of noise correlations between the laser modes, it is of utmost importance to have precise knowledge about the spectral behavior of the pump noise, which is the dominant source of noise in the frequency range of interest (10 kHz to 35 MHz). Moreover, it is found that the noise correlation also depends on how the spatially separated laser modes of the DF-VECSEL intercept the noise from a multimode fiber-coupled laser diode used for pumping both the laser modes. To this aim, a specific experiment is reported which aims at measuring the correlations between different spatial regions of the pump beam. The experimental results are in excellent agreement with a theoretical model based on modified rate equations

Influence of spin-dependent carrier dynamics on the properties of a dual-frequency vertical-external-cavity surface-emitting laser

The properties of a dual-frequency vertical external cavity surface emitting laser (VECSEL), in which two linear orthogonal polarization modes are oscillating simultaneously, are theoretically investigated. We derive a model based on the ideas introduced by San Miguel et al. [San Miguel, Feng, and Moloney, Phys. Rev. A 52, 1728 (1995)], taking into account the spin dynamics of the carriers inside the quantum well (QW) based gain medium of the dual-frequency VECSEL. This model is shown to succeed in describing quite a few properties of the dual-frequency VECSEL, such as the behavior of nonlinear coupling strength between the modes, the spectral behavior of intensity noises of the modes, and also the correlation between these intensity noises. A good agreement is found with experimental data. The variables associated with the spin dependent carrier dynamics can be adiabatically eliminated due to the class-A dynamical behavior of the considered laser which is based on a cm-long external cavity. This leads to a simple analytical description of the dynamics of the dual-frequency VECSEL providing a better understanding of the physics involved.

Optical Phase Sensitive Amplification for Microwave Photonics Applications: intermodulation distortion analysis

In the context of microwave photonics, we experimentally demonstrate the linearity of an optical phase sensitive amplifier by performing third order intermodulation distortion products measurements using two RF tones. Distortionless amplification is achieved.

Dual-frequency VECSELs for Microwave Photonics Applications

We describe how Vertical External Cavity Surface Emitting Lasers (VECSELs) can be used to distribute low-noise local oscillators in microwave photonics systems. In particular we discuss the peculiar noise properties of such dual-frequency lasers

Study of Intensity Noise Correlations in Dual-Frequency Nd:YAG Laser

The spectral behavior of the correlations between the intensity noises of the two linear-orthogonal-polarization modes of a dual-frequency Nd:YAG laser has been explored for different coupling strengths between the two modes.

Relaxation Oscillation Free Tunable Dual-Frequency VECSEL at Telecom Wavelength

Class-A tunable dual-frequency oscillation at 1.55 μm is demonstrated in a vertical-external-cavity surface emitting laser (VECSEL). The spectral behavior of the radio-frequency beatnote and the phase noise of the beatnote are also explored.