Development of compact and ultra-high-resolution spectrograph with multi-GHz optical frequency comb

摘要

In recent years, a calibration method for an astronomical spectrograph using an optical frequency comb (OFC) with a repetition rate of more than ten GHz has been developed successfully. But controlling filtering cavities that are used for thinning out longitudinal modes precludes long term stability. The super-mode noise coming from the fundamental repetition rate is an additional problem. We developed a laser-diode pumped Yb:Y_2O_3 ceramic oscillator, which enabled the generation of 4-GHz (maximum repetition rate of 6.7 GHz) pulse trains directly with a spectrum width of 7 nm (full-width half-maximum, FWHM), and controlled its optical frequency within a MHz level of accuracy using a beat note between the 4-GHz laser and a 246-MHz Yb-fiber OFC. The optical frequency of the Yb-fiber OFC was phase locked to a Rb clock frequency standard. Furthermore we also built a table-top multi-pass spectrograph with a maximum frequency resolution of 600 MHz and a bandwidth of 1 nm using a large-size high-efficiency transmission grating. The resolution could be changed by selecting the number of passes through the grating. This spectrograph could resolve each longitudinal mode of our 4-GHz OFC clearly, and more than 10% throughput was obtained when the resolution was set to 600 MHz. We believe that small and middle scale astronomical observatories could easily implement such an OFC-calibrated spectrograph.