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Spectrally-pure optical serrodyne modulation for continuously-tunable laser offset locking

Roame A. Hildebrand, Wance Wang, Connor Goham, Alessandro Restelli, Joseph W. Britton

Abstract

The comb-like spectrum added to laser light by an electro-optic modulator (EOM) finds use in a wide range of applications, including coherent optical communication, atomic spectroscopy, and laser frequency and phase stabilization. In some cases a sideband-free optical frequency shift is preferred, such as in laser offset locking using an optical cavity, single-photon frequency shifting, and laser range finding. Approaches to obtaining an optical frequency offset (OFO) involve trade-offs between shift range, conversion gain, and suppression of spurious sidebands. Here we demonstrate an OFO of continuous-wave 871 nm laser light by serrodyne modulation using a fiber EOM and radio-frequency (RF) tones from a commercial RF system on a chip (RFSoC) to achieve shifts of 40 to 800 MHz with > 15 dB suppression of spurious sidebands and < 1.5 dB conversion loss. We also observe a smoothly varying conversion gain. The utility of this tool is demonstrated by continuously shifting the offset of a cavity-locked laser from 50 to 1600 MHz, a capability useful in spectroscopy of unknown optical transitions.

Spectrally-pure optical serrodyne modulation for continuously-tunable laser offset locking

Abstract

The comb-like spectrum added to laser light by an electro-optic modulator (EOM) finds use in a wide range of applications, including coherent optical communication, atomic spectroscopy, and laser frequency and phase stabilization. In some cases a sideband-free optical frequency shift is preferred, such as in laser offset locking using an optical cavity, single-photon frequency shifting, and laser range finding. Approaches to obtaining an optical frequency offset (OFO) involve trade-offs between shift range, conversion gain, and suppression of spurious sidebands. Here we demonstrate an OFO of continuous-wave 871 nm laser light by serrodyne modulation using a fiber EOM and radio-frequency (RF) tones from a commercial RF system on a chip (RFSoC) to achieve shifts of 40 to 800 MHz with > 15 dB suppression of spurious sidebands and < 1.5 dB conversion loss. We also observe a smoothly varying conversion gain. The utility of this tool is demonstrated by continuously shifting the offset of a cavity-locked laser from 50 to 1600 MHz, a capability useful in spectroscopy of unknown optical transitions.

Paper Structure

This paper contains 12 sections, 3 equations, 8 figures, 1 table.

Table of Contents

  1. Introduction
  2. OFO Background and Implementations
  3. Serrodyne modulation
  4. Apparatus
  5. AMD Xilinx RFSoC 4x2 configuration
  6. Measurements
  7. Numerical modeling
  8. Performance at higher shift indices
  9. Application to PDH offset locking
  10. Conclusion
  11. Back matter
  12. References

Figures (8)

  • Figure 1: Experimental apparatus used for evaluating the phase-modulated spectrum. Blue lines indicate PM Fiber, yellow lines indicate non-PM fiber, red lines indicate free-space beam propagation, black lines indicate electrical connections.
  • Figure 2: Example traces of the synthesized serrodyne signal before and after the amplifier. Both signals are sampled at 50 GSPS here. The amplitude shown is slightly less than what would minimize the conversion loss at each frequency.
  • Figure 3: Example post-modulation spectra as observed on our optical spectrum analyzer.
  • Figure 4: OFO performance for N=1 as a function of the modulation frequency.
  • Figure 5: Measured DAC-to-optical transfer function $T_{\text{DTO}}$. To illustrate the the effect of this nonuniform transfer function, the plot in the bottom left shows the resulting phase modulation for a sampling-frequency-limited input saw wave at $300\text{ MHz}.$
  • ...and 3 more figures