The radial velocity or Doppler spectroscopy is one of the most important methods to detect exoplanets and investigate astrophysical processes. This technique has made immense success in discovering hundreds of planets with various masses since 1990 s, leading to the era of intensive discovery of exoplanets. Typically, as the mass of exoplanets decreases, a higher precision of the radial velocity is required. It was estimated that detecting an earth-size planet around a sun-like star requires at least 10 cm/s precision over timescales of a year, which demands high precision and high stability wavelength calibration technology. However, the widely used wavelength calibration technology based on atomic absorption cells and hollow cathode lamps could not support 10 cm/s measurement precision due to the influence of a series of factors such as uneven distribution of spectral lines and spectral line broadening. The development of laser technology, especially the laser frequency comb, opens new horizons to the high precision wavelength calibration. In 2008, researchers brought an astro-comb system based on mode-locked laser generating high repetition frequency (> 10 GHz) comb teeth which could be resolved clearly by spectrometer as a brand-new calibration source. The test results suggested that wavelength calibration with astro-comb had the potential to realize cm/s level precision, and in the next few years, the astro-comb system has been rapidly developed and further improved, gradually becoming the most successful high-precision wavelength calibration system available. Meanwhile, other high precision wavelength calibration technologies were also reported, including white light Fabry Perot etalon, electro-optic modulated astro-comb, micro resonator based astro-comb. These techniques also are the potential candidates to support 10 cm/s precision measurement, and are superior to the astro-comb system based on mode-locked laser in some aspects, such as system complexity and cost. Unlike the last generation techniques using atomic absorption cell and hollow cathode lamp, the calibration spectra of new generation techniques provide massive and dense peaks in frequency domain, which overcomes the defect of uneven distribution of atomic spectral lines. Nevertheless, there are still some issues that need to be addressed in order to achieve long-term wavelength calibration with 10 cm/s precision. The mode-locked laser astro-comb system inheriting high precision and high system complexity, more user-friendly turnkey system and wider frequency coverage are the focus of future development. On the contrary, although the white light Fabry Perot etalon has low system complexity, its absolute frequency acquisition of the transmission peaks and long-term stability need to be further improved. For the electro-optical modulated astro-comb and micro resonator based astro-comb, they could directly generate high repetition frequency combs without complicate filter system required in mode-locked laser astro-comb, but at present, their main working bands are in the near-infrared, and the visible wavelength coverage will be the main direction in the future. With the development of high-precision wavelength calibration techniques, it is foreseeable that the precision, long-term stability and coverage frequency will be improved, which would guarantee long-term wavelength calibration precision of 10 cm/s and meet the requirement of finding earth-analogs within the habitable zone of stars. In this paper, we summarized and illustrated the development and current status of wavelength calibration techniques using iodine absorption cells and Th-Ar lamps, as well as the new generation of high precision wavelength calibration techniques based on astro-combs and etalons. The basic principles and typical systems of new generation techniques are presented, their advantages and disadvantages are analyzed, and the future development prospects.
