Ke-Xun Sun

All-reflective Michelson, Sagnac, and Fabry-Perot interferometers based on grating beam splitters.

All-reflective Michelson, Sagnac, and Fabry-Perot interferometers based on grating beam splitters are experimentally demonstrated at a wavelength of 1064 nm. A 1200-groove/mm grating diffracting 0 and -1 orders with an efficiency of 48.2% for each order was used as a near-50/50 beam splitter. The all-reflective Sagnac and Michelson interferometers were formed by reintroducing both of the diffracted beams back to the grating. The Fabry-Perot interferometer was formed in a Littrow configuration by using a 1700-groove/mm grating with a blazing efficiency of 91% as a cavity coupler. These interferometers encompass all the fundamental configurations of all-reflective laser interferometric gravitational-wave detectors, promising improved wave-front quality by avoiding volume thermal effects in transmissive optics under high-power laser illumination.

LED deep UV source for charge management of gravitational reference sensors

Proof mass electrical charge management is an important functionality for the ST-7-LTP technology demonstration flight and for LISA. Photoemission for charge control is accomplished by using deep ultraviolet (UV) light to excite photoelectron emission from an Au alloy. The conventional UV source is a mercury vapour lamp. We propose and demonstrate charge management using a deep UV light emitting diode (LED) source. We have acquired selected AlGaN UV LEDs, characterized their performance and successfully used them to realize charge management. The UV LEDs emit at a 257 nm central wavelength with a bandwidth of ~12 nm. The UV power for a free-space LED is ~120 µW, and after fibre coupling is ~16 µW, more than sufficient for LISA applications. We have directly observed the LED UV light-induced photocurrent response from an Au photocathode and an Au-coated GRS/ST-7 proof mass. We demonstrated fast switching of UV LEDs and associated fast changes in photocurrent. This allows modulation and continuous discharge to meet stringent LISA disturbance reduction requirements. We propose and demonstrate AC charge management outside the gravitational wave signal band. Further, the megahertz bandwidth for UV LED switching allows for up to six orders of magnitude dynamic power range and a number of novel modes of operations. The UV LED based charge management system offers the advantages of small-size, lightweight, fibre-coupled operation with very low power consumption.

Advanced gravitational reference sensor for high precision space interferometers

LISA and the next generation of space-based laser interferometers require gravitational reference sensors (GRS) to provide distance measurements to picometre precision for LISA, and femtometre precision for the proposed Big Bang Observatory (BBO). We describe a stand-alone GRS structure that has the benefits of higher sensitivity and ease of fabrication. The proposed GRS structure enables high precision interferometric links in three-dimensional directions. The GRS housing provides the optical reference surface onto which the transmitted laser beam, and the independent received laser beam are referenced. The stand-alone GRS allows balanced optical probing of the distance of the proof mass relative to the housing at a power and wavelength that differ from the transmitted and received wavelengths and with picometre sensitivity without radiation pressure imbalance. The single parameter that reduces proof mass disturbance forces is the gap spacing. Optical readout allows the use of a large gap between the GRS housing and proof mass. We propose using rf-modulated optical interferometry to measure both relative displacement and absolute distance. Further we propose to use a reflective grating beamsplitter within the GRS and on the external optical bench. The reflective grating design eliminates the in-path transmissive optical components and the dn/dT related optical path effects, and simplifies the optical bench structure. Inside the GRS, a near-Littrow mounted grating enables picometre precision measurement at microwatts of optical power. Preliminary experimental results using a grating beamsplitter interferometer are presented, which demonstrate an optical sensing sensitivity of 30 pm Hz−1/2.

Polarization-based balanced heterodyne detection method in a Sagnac interferometer for precision phase measurement

We describe a balanced heterodyne detection method for a Sagnac interferometer that uses a polarization-dependent beam splitter. The signal and the local oscillator are orthogonally polarized components of a single laser beam, permitting the detection of the signal by subtraction of two photocurrents produced in appropriate polarization projections. Using this scheme, we experimentally demonstrate a phase measurement with a sensitivity of 9x10(-10) rad/ radicalHz. The measurement is robust in the presence of laser frequency noise, as a result of preserving the common-path nature of the Sagnac interferometer, and of laser-amplitude noise, as a result of balanced detection.

Balanced heterodyne signal extraction in a postmodulated Sagnac interferometer at low frequency

We describe a balanced-heterodyne postmodulated Sagnac interferometer signal extraction method that is suitable for gravitational wave detection. The method is simple to implement by placement of a polarization-selective modulator after the beam splitter in the dark port of the interferometer. The postmodulated Sagnac interferometer retains its common path advantage and exhibits insensitivity to laser frequency noise below, at, and above the heterodyne frequency. Balanced detection reduces sensitivity to laser amplitude noise. In this scheme mirror displacement signals were rf demodulated and observed from 0.2 to 10kHz.

Precise diffraction efficiency measurements of large-area greater-than-99%-efficient dielectric gratings at the Littrow angle

We have developed improved cavity-finesse methods for characterizing the diffraction efficiencies of large gratings at the Littrow angle. These methods include measuring cavity length with optical techniques, using a Michelson interferometer to calibrate piezoelectric transducer nonlinearities and angle-tuning procedures to confirm optimal alignment. We used these methods to characterize two 20 cm scale dielectric gratings. The values taken from across their surfaces collectively had means and standard deviations of micro=99.293% and sigma=0.164% and micro=99.084% and sigma=0.079%. The greatest efficiency observed at a single point on a grating was (99.577+/-0.002)%, which is also the most accurate measurement of the diffraction efficiency in the literature of which we are aware. These results prove that a high diffraction efficiency with low variation is achievable across large apertures for gratings.

UV LED operation lifetime and radiation hardness qualification for space flights

We report measurements of ultraviolet light emitting diode (UV LED) performance under conditions simulating operation in an orbiting satellite. UV LED light output maintained within less than 3% observational uncertainty, over more than 19,000 hours of operation in a nitrogen atmosphere, and over 8,000 hours operation at a pressure of less than 10-7 torr vacuum. In addition, irradiation with 63 MeV protons to a total fluence of 2×1012 protons/cm2 does not degrade the UV light output. Spectrally, the emissive center-wavelength and spectral shape are unchanged after proton irradiation within the precision of our measurement. These results qualify the UV LED operation lifetime and radiation hardness for space flights

Fiber-coupled, Littrow-grating cavity displacement sensor

We have demonstrated a compact, optical-fiber-fed, optical displacement sensor utilizing a Littrow-mounted diffraction grating to form a low-finesse Fabry-Perot cavity. Length changes of the cavity are read out via the Pound-Drever-Hall rf modulation technique at 925 MHz. The sensor has a nominal working distance of 2 cm and a total dynamic range of 160 nm. The displacement noise floor was less than 3x10(-10) m/sqrt[Hz] above 10(-2) Hz, limited by the frequency drift of the reference laser. A frequency-stabilized laser would reduce the noise floor to below 10(-12) m/sqrt[Hz]. The use of a 925 MHz modulation frequency demonstrates high-precision readout of a low-finesse compact resonant cavity.

Modular gravitational reference sensor development

The Modular Gravitational Reference Sensor (MGRS) is targeted as a next generation core instrument for both space gravitational wave detection and an array of other precision gravitational experiments in space. The objectives of the NASA funded program are to gain a system perspective of the MGRS, to develop key component technologies, and to establish important test platforms. Our original program was very aggressive in proposing ten areas of research and development. Significant advancements have been made in these areas, and we have met or exceeded the goals for the program set in 2007-2008. Additionally, we have initiated research projects for innovative technologies beyond the original plan. In this paper we will give a balanced overview of progress in MGRS technologies: the two layer sensing and control scheme, trade-off studies of GRS configurations, multiple optical sensor signal processing, optical displacement and angular sensors, differential optical shadow sensing, diffractive optics, proof mass center of mass and moment of inertia measurement, UV LED charge management, proof mass fabrication, thermal control and sensor development, characterization for various proof mass shapes, and alternative charge manage techniques.

Progress in Developing the Modular Gravitational Reference Sensor

Modular Gravitational Reference Sensor (modular GRS) was proposed by the Stanford Team in 2004. In a modular GRS, the laser beam from the remote the sensor does not illuminate the proof mass directly. The internal measurement from the housing to proof mass is separated from the external interferometry. A double‐sided grating further simplifies the structure and may better preserve the measurement precision. We review the recent progress in developing the modular GRS at Stanford. We are developing optical sensors with picometer resolution, capable of operating with a large gap for high precision readout. We have conducted an initial experiment incorporating RF heterodyne detection and thus lowered the optical power compared with direct detection. We have demonstrated sub‐nanoradian sensitivity of a grating angular sensor. We have successfully demonstrated fabrication of localized grating patterns on dielectric and gold surfaces. We have made critical progress in optical measurement of the mass center (MC) ...