Narrow-linewidth single-frequency lasers play an important role in coherent optical communication, coherent laser radar, microwave photonics, and fiber-optic sensing. The intensity noise of a single-frequency laser is an important indicator of its performance. Accurate evaluation of the intensity noise is of great significance, as it is necessary for optimizing laser performance, as well as for promoting and improving the design of application systems. However, current intensity noise measurement systems have insufficient performance in frequency bands and background noise and cannot meet the measurement requirements of advanced single-frequency lasers. Therefore, it is necessary to develop a relative intensity noise (RIN) measurement system with an ultralow broadband measurement background to satisfy the measurement requirements of more advanced single-frequency lasers and application systems.
In this study, the noise mechanism and RIN measurement methods are analyzed. Measurement errors in the system are calculated, including shot-noise from photodetectors, thermal noise generated by components, thermal noise in spectrum analysis, and calibration error. Numerical simulations of the main noise sources in the measurement system (see Fig.2) are conducted. A relationship that shows an increase in the photoelectric current can simultaneously reduce the shot-noise limit and the thermal noise limit of the measurement system is observed. A method is proposed for reducing the shot-noise and thermal noise limits of the system by generating a high photocurrent from a photodetector and combining it with a low-noise spectrum analyzer. On this basis, a measurement system of 40 kHz to 40 GHz is built with a background noise of -171 dBc/Hz.
The above measurement principles and methods are experimentally verified using an ultralow background noise measurement system (see Fig.3). An Emcore 1782 distributed-feedback semiconductor laser diode (DFB LD) is used to generate a photocurrent of 1 to 40 mA through an attenuator to measure the RIN. When the photocurrent is less than 10 mA, the measured noise power spectral density remains unchanged, and the measurement results are limited by the system background noise. When the photocurrent is 40 mA, the measured noise power spectral density is significantly higher than the background noise power spectral density, clearly reflecting the noise characteristics of the laser. Under a 40 mA photocurrent, the measurement results in the 100 MHz to 1 GHz frequency band reached a shot-noise limit of -171 dBc/Hz, which is consistent with the theoretical analysis. Subsequently, the feasibility of using an erbium-doped fiber amplifier (EDFA)-amplifying photocurrent to measure the RIN is analyzed (see Fig.4). The results show that when the photocurrent is insufficient, the EDFA can amplify the optical power, increase the photocurrent, and reduce the background noise, thereby making the measurement value closer to the true noise level of the laser. However, this process introduces additional noise that makes the measured value larger than the actual noise value. Ultralow RIN lasers are then measured and characterized using this process. The PLANEX series planar-waveguide external cavity diode laser (PWECL) produced by RIO Lasers, the 1782 DFB LD produced by Emcore, and the self-developed nonplanar ring oscillator solid-state laser (NPRO) display better performance parameters compared with the manufacturer’s data under ultralow background RIN measurements (see Fig.5). The three measured lasers reached the corresponding shot-noise limits at 100 MHz?1 GHz. The results clearly demonstrate noise rollover, multiple relaxation oscillation peaks, intensity modulation harmonic distortion (see Fig.7), and other rich experimental phenomena. Thus, the effectiveness of the ultralow background noise measurement method is confirmed by experiments.
This study aimed to accurately measure the RIN of single-frequency laser sources in high-speed communication and microwave photonic systems. A method for achieving ultralow background RIN measurements of single-frequency laser sources is proposed, using a high-current photodetector to improve the photocurrentand a low-noise spectrum analyzer to reduce the thermal noise. An ultralow background RIN measurement system is built with a spectrum analysis frequency band of 40 GHz and a measurement background noise of -171 dBc/Hz. Subsequently, the RIN characteristics of common optical communication processes are characterized, clearly demonstrating noise rollover, multiple relaxation oscillation peaks, intensity-modulated harmonic distortion, and other characteristics of typical laser sources at extremely low frequencies. The research results have important application prospects for laser performance design optimization and evaluation.
