I. Introduction

II. Related Surveys and Our Contributions

III. Evolution Toward 6G

IV. 6G Fronthaul Interface and Main Splitting Options

V. Motivation for 6G Optical Fronthaul

VI. Cutting-Edge Research Related to 5G/6G Optical Fronthaul

VII. Research Projects Related to 5G/6G Fronthaul

VIII. Future Research Directions

IX. Conclusions