Compact narrow linewidth 1.5-μm frequency references for laser diode frequency stabilization

In several previous works about 1.5-μm laser diode frequency stabilization, the best laser frequency stabilities were limited to δv/v ≈ 10^-10 by the lack of narrow linewidth 1.5 μm frequency references. Although numerous molecular absorption lines are found in this wavelength region, their ≈500-MHz-wide Doppler limited absorptions were too large to obtain a laser diode frequency reference with a high stability. We addressed this issue recently and obtained the first 2-MHz-wide 1.54-μm saturated absorption line in acetylene using a simple buildup cavity method which is well suited to the practical construction of a frequency stabilized 1.5-μm laser diode. As shown in Fig. 1, a 20-cm-long confocal Fabry-Perot cavity filled with the absorbing gas at low pressure was used as a frequency reference. With a cavity finesse of about 100, a 8 mW DFB laser was enough to obtain the saturated absorption. However, several practical applications of such frequency stabilized lasers are linked to optical communications which may require a 1.56-μm wavelength in order to be compatible with optical fibers minimum absorption wavelength. Therefore, we show here that this technique is also applicable without modifications to HCN gas, which has several strong absorption lines at 1.56 μm. However, as no 1.56-μm lasers were available, we demonstrated this possibility with a very similar 1.54-μm HCN line: in Fig. 2, we show a 5-MHz-wide HCN saturated absorption line which has been obtained with the same experimental apparatus. Another important topic that we have investigated for practical purpose is the use of high finesse cavities which allow, either to increase the detection sensitivity by several orders of magnitude, or to decrease the size of the device. In this framework, we also demonstrated a compact version of such frequency references: a 25-mm long high finesse (F > 10,000) reference cell filled with C₂H₂ provided a 2.8 MHz wide absorption line with a large 140-mV signal corresponding to 4% of total transmitted power, and a very good S/N. The obtained line, shown in Fig. 3, is almost perfectly fitted by a Lorentzian lineshape and the observed linewidth is probably limited by power broadening. It must be noticed that the linewidth of these frequency references are mainly instrumentally limited and could be well improved by further design improvements. The frequency stabilization of laser diodes using this technique is underway and should lead to a 2 orders of magnitude improvement of current 1.5-μm laser diode frequency standards. These achievements open the way to new absolute laser frequency standards in the 1.5-μm wavelength region: for that purpose, we are now considering the absolute measurement of such narrow molecular lines using an existing laser reference frequency.