Jeffrey Livas

Laser Frequency Stabilization and Control through Offset Sideband Locking to Optical Cavities

We describe a class of techniques whereby a laser frequency can be stabilized to a fixed optical cavity resonance with an adjustable offset, providing a wide tuning range for the central frequency. These techniques require only minor modifications to the standard Pound-Drever-Hall locking techniques and have the advantage of not altering the intrinsic stability of the frequency reference. We discuss the expected performance and limitations of these techniques and present a laboratory investigation in which both the sideband techniques and the standard, on-tunable Pound-Drever- Hall technique reached the 100Hz/square root(Hz) level.

LASER RANGING FOR GRAVITATIONAL, LUNAR AND PLANETARY SCIENCE

More precise lunar and Martian ranging will enable unprecedented tests of Einsteins theory of general relativity as well as lunar and planetary science. NASA is currently planning several missions to return to the Moon, and it is natural to consider if precision laser ranging instruments should be included. New advanced retroreflector arrays at carefully chosen landing sites would have an immediate positive impact on lunar and gravitational studies. Laser transponders are currently being developed that may offer an advantage over passive ranging, and could be adapted for use on Mars and other distant objects. Precision ranging capability can also be combined with optical communications for an extremely versatile instrument. In this paper we discuss the science that can be gained by improved lunar and Martian ranging along with several technologies that can be used for this purpose.

Characterization of photoreceivers for LISA

LISA will use quadrant photoreceivers as front-end devices for the phasemeter measuring the motion of drag-free test masses in both angular orientation and separation. We have set up a laboratory testbed for the characterization of photoreceivers. Some of the limiting noise sources have been identified and their contribution has been either measured or derived from the measured data. Wehavebuiltaphotoreceiverwitha0.5mmdiameterquadrantphotodiodewith an equivalent input current noise of better than 1.8pA Hz −1/2 below 20 MHz

Note: Silicon Carbide Telescope Dimensional Stability for Space-based Gravitational Wave Detectors

Space-based gravitational wave detectors are conceived to detect gravitational waves in the low frequency range by measuring the distance between proof masses in spacecraft separated by millions of kilometers. One of the key elements is the telescope which has to have a dimensional stability better than 1 pm Hz(-1/2) at 3 mHz. In addition, the telescope structure must be light, strong, and stiff. For this reason a potential telescope structure consisting of a silicon carbide quadpod has been designed, constructed, and tested. We present dimensional stability results meeting the requirements at room temperature. Results at -60 °C are also shown although the requirements are not met due to temperature fluctuations in the setup.

Frequency-tunable pre-stabilized lasers for LISA via sideband locking

Laser frequency noise mitigation is one of the most challenging aspects of the LISA interferometric measurement system. The unstabilized frequency fluctuations must be suppressed by roughly 12 orders of magnitude in order to achieve stability sufficient for gravitational wave detection. This enormous suppression will be achieved through a combination of stabilization and common-mode rejection techniques. The stabilization component will itself be achieved in two stages: pre-stabilization to a local optical reference followed by arm locking to some combination of the inter-spacecraft distances. In order for these two stabilization stages to work simultaneously, the lock-point of the pre-stabilization loop must be frequency tunable. The current baseline stabilization technique, Pound–Drever–Hall locking to an optical cavity, does not provide tunability between cavity resonances. Here we present a modification to the baseline technique that allows the laser to be locked to a cavity resonance with an adjustable frequency offset. This technique requires no modifications to the optical cavity itself, thus preserving the stability of the frequency reference. We present measurements of the system performance and demonstrate that the offset locking techniques are compatible with arm locking.

Implementation of armlocking with a delay of 1 second in the presence of Doppler shifts

LISA relies on several techniques to reduce the initial laser frequency noise in order to achieve an interferometric length measurement with an accuracy of ≈ 10pm/. LISA will use ultra-stable reference cavities as a first step to reduce the laser frequency noise. In a second step the frequency will be stabilized to the LISA arms which provide a better reference in the frequency band of interest. We present experimental results demonstrating Arm locking with LISA-like light travel times and Doppler shifts. We also integrated this system with a LISA-like pre-stabilization system using our ultra-stable cavities. The addition of realistic Doppler shifts led to further refinements of the arm locking controllers compared to the controller architecture discussed in the past. A first experimental result of the new controller is also presented.

Carbon fiber reinforced polymer dimensional stability investigations for use on the laser interferometer space antenna mission telescope

The laser interferometer space antenna (LISA) is a mission designed to detect low frequency gravitational waves. In order for LISA to succeed in its goal of direct measurement of gravitational waves, many subsystems must work together to measure the distance between proof masses on adjacent spacecraft. One such subsystem, the telescope, plays a critical role as it is the laser transmission and reception link between spacecraft. Not only must the material that makes up the telescope support structure be strong, stiff, and light, but it must have a dimensional stability of better than 1 pm Hz(-1/2) at 3 mHz and the distance between the primary and the secondary mirrors must change by less than 2.5 μm over the mission lifetime. Carbon fiber reinforced polymer is the current baseline material; however, it has not been tested to the pico meter level as required by the LISA mission. In this paper, we present dimensional stability results, outgassing effects occurring in the cavity and discuss its feasibility for use as the telescope spacer for the LISA spacecraft.

Optical telescope design for a space-based gravitational-wave mission

Space-based gravitational-wave observatories will systematically study the source-rich band of gravitational waves from 0.0001 Hz to 1 Hz. All current designs require propagation of a laser beam from one spacecraft to another over immense distances. An optical telescope is needed for efficient power delivery and its design is driven by the interferometric displacement sensitivity requirements. Here we describe the design for a catoptric telescope that meets those requirements, emphasizing differences from the usual specifications for high quality image formation, and discuss design trade-offs as well as early results from research into scattered light suppression and modeling that may enable alternative designs.

Ultra-low noise large-area InGaAs quad photoreceiver with low crosstalk for laser interferometry space antenna

Quad photoreceivers, namely a 2 x 2 array of p-i-n photodiodes followed by a transimpedance amplifier (TIA) per diode, are required as the front-end photonic sensors in several applications relying on free-space propagation with position and direction sensing capability, such as long baseline interferometry, free-space optical communication, and biomedical imaging. It is desirable to increase the active area of quad photoreceivers (and photodiodes) to enhance the link gain, and therefore sensitivity, of the system. However, the resulting increase in the photodiode capacitance reduces the photoreceivers bandwidth and adds to the excess system noise. As a result, the noise performance of the front-end quad photoreceiver has a direct impact on the sensitivity of the overall system. One such particularly challenging application is the space-based detection of gravitational waves by measuring distance at 1064 nm wavelength with ~ 10 pm/√Hz accuracy over a baseline of millions of kilometers. We present a 1 mm diameter quad photoreceiver having an equivalent input current noise density of < 1.7 pA/√Hz per quadrant in 2 MHz to 20 MHz frequency range. This performance is primarily enabled by a rad-hard-by-design dualdepletion region InGaAs quad photodiode having 2.5 pF capacitance per quadrant. Moreover, the quad photoreceiver demonstrates a crosstalk of < -45 dB between the neighboring quadrants, which ensures an uncorrected direction sensing resolution of < 50 nrad. The sources of this primarily capacitive crosstalk are presented.

Arm locking for the Laser Interferometer Space Antenna

The Laser Interferometer Space Antenna mission is a planned gravitational wave detector consisting of three spacecraft in heliocentric orbit. Laser interferometry is used to measure distance fluctuations between test masses aboard each spacecraft to the picometer level over a 5 million kilometer separation. Laser frequency fluctuations must be suppressed in order to meet the measurement requirements. Arm-locking, a technique that uses the constellation of spacecraft as a frequency reference, is a proposed method for stabilizing the laser frequency. We consider the problem of arm-locking using classical optimal control theory and find that our designs satisfy the LISA requirements.