Iodine-stabilized Nd:YVO/sub 4/ laser with digital controller

Abstract

Iodine-stabilized lasers are generally used as optical frequency standards and (primary) length standards. Especially the iodine-stabilized helium-neon laser has been developed extensively for this purpose during the last decades. In more recent years, a variety of lasers locked to atomic, molecular and ion transitions have been developed for several applications, like multi-color interferometry, or as optical frequency standards.We have equipped an iodine-stabilized Nd:YVO4 laser[1l] with a digital controller to stabilize the laser frequency to molecular iodine. With intracavity frequency doubling using a KTP crystal, a 532 nm output is generated. The laser frequency can be tuned over a large range by temperature tuning of the KTP crystal, which acts as a Lyot filter. Furthermore, the laser frequency can be tuned over a range of about 15 GHz with a piezo-actuator (PZT) . Another PZT is used for stabilization of the frequency. The laser is stabilized to saturated molecular iodine using Doppler-free spectroscopy and 3f modulation.For the frequency stabilization we use a stand-alone programmable digital signal processor (DSP, Keithley instruments) interfaced to a PC. The stabilization process is fully performed by the DSP, which has a clock speed of 40 MHz and is equipped with several 16 bit AD and DA converters. A modulation signal with a frequency of 1 kHz is generated with 60 points per period. Demodulation, filtering and integration is performed digitally, resulting in an error signal for frequency locking of the laser to a hyperfine component of an iodine transition. A user interface has been developed for parameters setting (phase, modulation width, etc.) and data acquisition. The approach with the digital controller provides great flexibility. For example, we implemented a scope mode , in which the laser frequency is continuously sweeping over an iodine transition. In this mode the laser can be locked to one of the hyperfine components, simply by selecting it with a single mouse-click.A typical scan of the R (56) 32-0 transition (line 1110) is shown in the graph below. 60 ._ n 30 C.a-2 _(A O =E 0 . -30 s0 I -60 16000 17000 18000 19000 DAC units Fig. 1 Third harmonic response ofa frequency sweep over the (R56)32-0 transition. The full width of this scan is about 1 GHz. The temperature of the coldfinger of the iodine cell is S C. With temperature tuning of the KTP crystal the total scan range of the laser exceeds 150 GHz, so a wide range of iodine transitions can be addressed. Preliminary measurements show that the aJO component of the recommended transition (line 1110) agrees within a few kHz with the values from literature[2]. An accuracy of better than 10-11 is thus easily achieved and further improvement of the long-term stability is expected to be achieved in near future.We successfully applied the stabilized Nd:YVO4 laser for measurement of gauge blocks (up to 1000 mm), showing that the digitally controlled laser is a suitable as a source for dimensional interferometric measurements.In conclusion, we dev...