Free-space optical communication holds great promise for future space missions requiring high data rates. For data communication in deep space, the current architecture employs pulse position modulation (PPM). In this scheme, the light is transmitted and detected as pulses within an array of time slots. While the PPM method is efficient for data transmission, the phase of the laser light is not utilized.
The phase coherence of a PPM optical signal has been investigated with the goal of developing a new laser communication and ranging scheme that utilizes optical coherence within the established PPM architecture and photon- counting detection (PCD). Experi - mental measurements of a PPM modulated optical signal were conducted, and modeling code was developed to generate random PPM signals and simulate spectra via FFT (Fast Fourier Transform) analysis. The experimental results show very good agreement with the simulations and confirm that coherence is preserved despite modulation with high extinction ratios and very low duty cycles.
A real-time technique has been developed to recover the phase information through the mixing of a PPM signal with a frequency-shifted local oscillator (LO). This mixed signal is amplified, filtered, and integrated to generate a voltage proportional to the phase of the modulated signal. By choosing an appropriate time constant for integration, one can maintain a phase lock despite long “dark” times between consecutive pulses with low duty cycle. A proof-of-principle demonstration was first achieved with an RF-based PPM signal and test setup. With the same principle method, an optical carrier within a PPM modulated laser beam could also be tracked and recovered. A reference laser was phase-locked to an independent pulsed laser signal with low-duty-cycle pseudo-random PPM codes. In this way, the drifting carrier frequency in the primary laser source is tracked via its phase change in the mixed beat note, while the corresponding voltage feedback maintains the phase lock between the two laser sources.
The novelty and key significance of this work is that the carrier phase information can be harnessed within an optical communication link based on PPM-PCD architecture. This technology development could lead to quantum-limited efficient performance within the communication link itself, as well as enable high-resolution optical tracking capabilities for planetary science and spacecraft navigation.
This work was done by David C. Aveline, Nan Yu, and William H. Farr of Caltech for NASA’s Jet Propulsion Laboratory. NPO-47994
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Optical Phase Recovery and Locking in a PPM Laser Communication Link
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Overview
The document titled "Optical Phase Recovery and Locking in a PPM Laser Communication Link" from NASA's Jet Propulsion Laboratory outlines advancements in coherent optical communication technologies, specifically focusing on pulse position modulation (PPM) for deep space applications. The primary objective is to enhance spacecraft navigation accuracy through improved optical Doppler tracking and ranging capabilities.
Key highlights include the development of an optical phase lock loop (PLL) that can maintain phase coherence at extremely low power levels, down to femto-watt (fW) levels. This capability is crucial for deep space missions where signals may take significant time to return, necessitating a stable phase reference. The document reports successful demonstrations of phase lock stability, achieving a phase slip rate of less than one cycle-slip per second at optical powers as low as 40 femtoWatts.
The research emphasizes the significance of PPM architecture, which encodes data by modulating the amplitude of light pulses based on their arrival times. This method allows for efficient use of bandwidth and improves the robustness of communication links in challenging environments. The power spectrum analysis reveals characteristic sinc envelopes and sharp peaks corresponding to the modulation scheme, indicating effective signal processing.
Further investigations are underway to characterize phase lock stability and assess average power limitations related to the PPM duty cycle. The document also discusses the integration of photon counting detection to test coherence within the PPM framework, aiming for quantum-limited performance in optical communications.
The findings presented in this document are part of a broader effort to develop advanced communication systems for space exploration, with potential applications extending beyond aerospace to various technological and commercial fields. The research underscores the importance of maintaining optical phase coherence for precision tracking and ranging, which is vital for future missions that require high accuracy in navigation and data transmission.
Overall, this technical support package serves as a comprehensive overview of the ongoing research and development efforts at NASA's JPL, highlighting the innovative approaches being taken to enhance optical communication capabilities in deep space environments.
