text: The unique features of NICE-OHMS, in particular its high sensitivity, imply that it has a large potential for a variety of applications. First developed for frequency standard applications, with an astonishing detectability of 10 cm, it has later been used for spectroscopic investigations as well as chemical sensing and trace species detection, with detectabilities in the 10 - 10 cm range. However, although the NICE-OHMS technique has shown to possess an extremely high detectability, it has so far only sparsely been developed towards trace gas analysis.
  One of the biggest hurdles for implementation of the NICE-OHMS technique is indisputably the locking of the frequency of the laser to that of a cavity mode. Although the requirements for the performance of the lock are less stringent than for other direct cw-CEAS techniques (due to the noise-immune principle), the laser frequency still has to be kept locked within the cavity mode during signal acquisition, i.e. it should follow the mode while the cavity is scanned, including a possible wavelength modulation. It can be difficult to achieve these goals if the free-running linewidth of the laser is significantly larger than the cavity mode width and if the laser is prone to sudden frequency excursions due to technical noise from the surroundings. This is usually the case when working with medium- or high finesse cavities (with transmission mode widths in the low kHz range) and standard types of lasers, e.g. external cavity diode lasers (ECDLs), with free-running linewidths in the MHz range. Electronic feedback loops with high bandwidths (typically a few MHz) and high gain are then needed to couple a substantial amount of the laser power into a cavity mode and to ensure stable performance of the lock.
  With the advent of narrow linewidth fiber lasers, the problems connected to laser locking can be significantly reduced. Fiber lasers with free-running linewidths as narrow as 1 kHz (measured over a fraction of a second), thus two to three orders of magnitude below those of ECDLs, are available today. Evidently, this feature simplifies the feedback electronics (bandwidths as low as 10 kHz are sufficient) and the locking procedure considerably. Moreover, the design and working principle of fiber lasers make them less affected by external disturbances, e.g. mechanical and acoustical noise, than other solid state lasers or ECDLs. In addition, the availability of integrated-optics components, such as fiber based electro-optic modulators (fiber EOMs), offers the possibility to further reduce the complexity of the setup. The first realizations of a NICE-OHMS system based upon a fiber laser and a fiber EOM have recently been demonstrated. It was shown that CH could be detected down to 4.5•10 atm (4.5 ppt) with an instrumentation that is very sturdy. It is clear that this has brought NICE-OHMS a step closer to become a practically useful technique for ultra-sensitive trace species detection!