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Length sensing and control of a Michelson interferometer with power recycling and twin signal recycling cavities

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Abstract

The techniques of power recycling and signal recycling have proven as key concepts to increase the sensitivity of large-scale gravitational wave detectors by independent resonant enhancement of light power and signal sidebands within the interferometer. Developing the latter concept further, twin signal recycling was proposed as an alternative to conventional detuned signal recycling. Twin signal recycling features the narrow-band sensitivity gain of conventional detuned signal recycling but furthermore facilitates the injection of squeezed states of light, increases the detector sensitivity over a wide frequency band and requires a less complex detection scheme for optimal signal readout. These benefits come at the expense of an additional recycling mirror, thus increasing the number of degrees of freedom in the interferometer which need to be controlled.

In this article we describe the development of a length sensing and control scheme and its successful application to a tabletop-scale power recycled Michelson interferometer with twin signal recycling. We were able to lock the interferometer in all relevant longitudinal degrees of freedom and thus laid the foundation for further investigations of this interferometer configuration to evaluate its viability for the application in gravitational wave detectors.

© 2013 Optical Society of America

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Figures (5)

Fig. 1
Fig. 1 General comparison of the dual recycling (left) and the power recycled twin signal recycling Michelson (right) topologies, both including squeezed light injection. Compared to dual recycling, the power recycled TSR interferometer features a second recycling mirror in the detection port, TSRM, which forms a three mirror coupled cavity together with the SRM and the Michelson end mirrors. For a broadband sensitivity improvement at shot noise limited frequencies by squeezing injection, the dual recycling interferometer with detuned SR requires an additional filter cavity, to compensate for the SR cavity-induced rotation of the squeezing ellipse. Contrasting this, the TSR configuration is inherently compatible with squeezing at shot noise limited frequencies and requires no additional optical filter in the squeezing path.
Fig. 2
Fig. 2 Left: Definition of the primary longitudinal degrees of freedom of the Michelson interferometer with power recycling and TSR cavities. The length degrees of freedom are expressed as phase changes acting on the light fields in the interferometer which are a result of changes of the mirror spacings. The parameter k = 2π/λ is the wave number. Right: Schematic drawing of the conceptual approach to length sensing in the TSR interferometer. The sensing scheme is based on two different pairs of phase modulation sidebands and four heterodyne signal extraction ports to obtain four error signals for feedback control of the longitudinal DOF of the interferometer. The half wave plate in the input beam path and the polarizing beam splitter between the Michelson beam splitter and the SRM permit interferometer locking with orthogonal polarization modes. The blue and green lines represent phase modulation sidebands at 15 MHz and 123.6 MHz, respectively, which co-propagate with the carrier field represented by the red line. Whereas the 15 MHz sidebands are reflected at the PRC and serve for the extraction of an error signal for the PRM position, the 123.6 MHz sidebands are resonant in the PRC and in the TSRC and are employed for the extraction of length signals for the remaining degrees of freedom.
Fig. 3
Fig. 3 Optical layout of the power recycled twin signal recycling interferometer experiment. A number of auxiliary optics were omitted for clarity.
Fig. 4
Fig. 4 Monitored photo detector DC transients, recorded during lock acquisition and fully locked operation of the interferometer. To make the different acquisition phases better distinguishable the time axis was stretched between the beginning of the measurement and t = 2 min. The traces show light powers measured in the symmetric port (i.e. in reflection of the power recycling cavity, blue trace), in the Michelson-internal pick-off port (orange trace), in the port in transmission of one of the Michelson end mirrors (green trace) and in the asymmetric port (i.e. in transmission of the TSRC, magenta trace). The gray shaded areas represent different stages of the acquisition sequence, resulting in a fully locked interferometer after t = 1.4 min. The proportional gains of the feedback loops were optimized at t = 2 min. The numbers 1–6 in the plots represent the different stages of lock acquisition.
Fig. 5
Fig. 5 Monitored correction signals generated by the servo controllers for the four longitudinal DOF. To make the different acquisition phases better distinguishable the time axis was stretched between the beginning of the measurement and t = 2 min. The traces were recorded simultaneously with the data shown in Fig. 4. These signals were amplified and in turn applied to piezoelectric transducers actuating on the power recycling mirror (blue trace), one of the Michelson end mirrors (orange trace), the signal recycling (green trace) and the twin signal recycling mirror (magenta trace), respectively. Long term drifts eventually led to a lock-loss of the instrument due to the limited range of the actuators. The numbers 1–6 in the plots represent the different stages of lock acquisition.

Tables (3)

Tables Icon

Table 1 Modulation/demodulation parameters of the signal extraction system. In the first column the longitudinal DOF are listed along with, in brackets, the mirrors which were actuated upon to tune the respective DOF. Whereas the 15 MHz sidebands are reflected at the PRC, the 123.6 MHz sidebands are resonant in the PRC and in the TSRC, cf. Fig. 2.

Tables Icon

Table 2 Normalized sensing matrix as obtained from the numerical model of the TSR interferometer. Rows correspond to different signal extraction ports, columns to different longitudinal DOF. For the calculation of each entry it was assumed implicitly that the other DOF were at their designated operating points. Heterodyne length signals which stem from unwanted coupling of neighboring DOF are highlighted in blue, numerical zeros are printed in green.

Tables Icon

Table 3 Design parameters of the power recycled TSR interferometer experiment.

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