Key Techniques Of Optical Frequency Synthesis

High frequency stability and narrow-linewidth lasers are indispensable tools in various scientific fields, such as tests of fundamental physics, precision spectroscopy, optical atomic clocks, precision measurement and metrology. Recently, state-of-the-art lasers with subhertz-linewidth at some specific wavelengths have been constructed by using the Pound-Drever-Hall (PDH) technique. In such laser systems, ultra-stable external optical cavities with high finesse are employed to reduce the laser linewidth. However, for most applications, in which narrow-linewidth lasers should be realized over a wide range of wavelength, it will become complex and much expensive. And it is also difficult to realize if a narrow-linewidth laser needs to be continuously tuned over a relatively broad spectrum precisely. Optical frequency synthesis is the feasible solution to satisfy requirements. In this thesis, we introduce a method of optical frequency synthesis which can yield narrow-linewidth lasers at an arbitrary wavelength over a relatively broad spectrum by transferring high frequency stability and coherence from an ultrastable laser at a specific wavelength to a tunable, single-frequency laser bridged by an optical frequency comb. Key techniques of optical frequency synthesis including a subhertz-linewidth laser, a narrow-linewidth optical frequency comb and a tunable, narrow-linewidth single-frequency laser have been studied and developed.Two narrow-linewidth Nd:YAG lasers at1064run are constructed by independently frequency-stabilizing to two separately-located, vertically-mounted vibration-insensitive reference cavities of7.75cm-long. Benefiting from passive vibration isolation, high stable temperature control with sub-mK stability, fiber noise cancellation system, as well as stable optical systems, we have realized two subhertz-linewidth lasers. Measurements show that each laser system has achieved a linewidth of0.6Hz and fractional frequency instability of1.2×10-15at1s averaging time. Systematic evaluation shows the frequency instability and linewidth of each laser system is limited by thermal noise of the reference cavity. The laser systems have been running continuously for more than one month.By precisely phase-locking an optical frequency comb with a repetition rate of800MHz to one of the subhertz-linewidth NdrYAG lasers at1064nm, we have realized a narrow-linewidth optical frequency comb. The absolute linewidth of the comb teeth has been measured to be1Hz over an octave spectrum from532nm to1064nm.In order to obtain a frequency tunable, narrow-linewidth untra-stable laser, a widely tunable, single-frequency Ti:sapphire laser is precisely phase-locked to the narrow-linewidth optical frequency comb with a frequency tracking accuracy of8×10-17at1s averaging time and a relative linewidth of less than1mHz. Measurements show that a narrow-linewidth Ti:sapphire laser at the Hz level has been realized. The laser frequency of the narrow-linewidth Ti:sapphire laser can be coarsely determined by a wavemeter and be finely tuned by precision phase-locking systems.Finally, an outlook of further work of optical frequency synthesis on laser frequency tuning by computer and spectrum extension has been discussed.