Direct Fiber Positioning System


[19-OCT-22] Our Direct Fiber Positioning System (DFPS) uses the slight bending of a piezo-electric cylinder to displace the tip of an optical fiber at the end of a lever arm. Each fiber positioner consists of a cylindrical, piezo-electric actuator, a long tube for a mast, one or two polished fibers held in a ferrule, and a controller and amplifier to generate the actuator's electrode voltages. The DFPS provides one fiber positioner per 5 mm × 5 mm square area on the focal surface of a telescope. When combined with a spectrometer, the DFPS provides one measurement of red shift per 5 mm square. In order to make sure the fiber tips are centered upon their celestial targets, a fiber view camera (FVC) must be mounted some distance above the fiber tips in the telescope. We either back-illuminate the detector fibers so they glow in the view of the fiber view camera, or we will illuminate separate guide fibers that are mounted adjacent to the detector fibers. The fiber view camera confirms fiber positions before exposure. The sag and creep compensation systems of the DFPS ensure that the fiber remains on target during the exposure. Our work on the DFPS is supported entirely by a Phase I Small Business Initiative Research (SBIR) grant from the National Science Foundation (NSF, Grant Number 2111936). See our Development Log for our latest results.


Figure: Sketch of Direct Fiber Positioner. We show bending of actuator and translation of the mast tip. The inset shows a cross-section through the actuator.

By exaggerating the bending of the actuator, the sketch above shows how the mast and actuator move the fiber tip. In our current prototypes, the actuator is 40 mm long with maximum bending ±6.3 mrad. The mast is 300 mm long, so the movement of the fiber is ±1.9 mm. We can place the fiber anywhere in a 3.8-mm square, which is itself centered upon the 5.0-mm square footprint of the fiber positioner. If we were to increase the length of the mast to 400 mm, the fiber's range would cover a 5.0-mm square. We consider the complications of extending the mast in our development page.


Figure: Sketch of Fiber Spectrograph. In the front-end of the spectrograph, fiber positioners place fiber tips on the images of celestial objects. The fibers transport the light from these celestial objects to the back-end of the spectrograph, where prisms spread the light into spectra that we record with image sensors.

The DFPS is the front end of a multi-object spectrograph. The back end will be a system of diffraction gratings, mirrors, lenses, and image sensors that creates and records the spectra of the light from each fiber. The front end we propose to build will present two fibers at the tip of each positioner. One will act as a guide fiber, the other as a detector fiber. The guide fiber we illuminate from the far end so we can measure the location of the mast, the detector fiber we use to collect the light of a celestial object. The guide fibers we can illuminate and flash individually and at will, which simplifies our fiber vew camera optics, increases the precision with which we can locate the fibers, increases the speed with which we can calibrate the array, and simplifies the back end by eliminating the shutters that would be required for use of the detector fibers as their own guide fibers.

TermMeaning
PositionerThe combination of an actuator, mast, and controller that together move the tip of a fiber.
ActuatorThe piezo-electric cylinder that bends when we apply voltage to its electrodes.
MastThe long tube that acts as a lever arm to turn the bending of the actuator into translation of the fiber tip.
FerruleThe cylinder with a precision center hole that presents the polished fiber tip.
ControllerThe logic, converters, and amplifiers that generate a single actuator's four electrode voltages.
Base BoardThe printed circuit board that supports all the positioners of a single cell.
Service BoardThe printed circuit board that holds the fiber controllers for all the positioners of a single cell.
Detector CellA base board, its fibers, its positioners, its service board, and all its controllers.
Detector FiberA fiber used to capture and transport the light from a celestial object.
Guide FiberA fiber used to reveal the location of a detector fiber.
Fiber View CameraA camera looking down on the fiber tips.
Front EndThe multi-object detector: fibers, positioners, and the fiber view camera.
Back EndThe spectrometer itself: we plug the fibers into it and it records spectra.
Table Glossary of DFPS Terminology.

The photograph below shows our Test Stand One (TS1) with four fiber positioners. The actuator is a 40-mm long, 3.6-mm OD, 2.8-mm ID, piezo-electric cylinder with four electrodes to which we apply up to ±250 V in order to generate bending of ±6 mrad in two perpendicular directions. The mast is a 300-mm long, 2.40 mm OD, 2.25 mm ID stainless steel tube. We observe ±1.7 mm range of motion at fiber tip, a diagonal range of 4.8 mm. The positioner is soldered to four pins on the base board. The mast is fastened with epoxy to the end of the actuator. An optical fiber terminates in 1.25-mm diameter white zirconia ferrules at top and bottom. We connect another fiber to the base of the positioner with the help of a zirconia sleeve. We shine light into the far end of the fiber and watch the tip of the fiber with the fiber view camera. The camera allows us to measure the movements of the fiber with better than 5-μm precision.


Figure: A Four-Fiber Positioner. Each positioner consists of a 40-mm actuator, 300-mm steel mast, two 1.25-mm diameter zirconia ferrules and an optical fiber running down the middle. The fiber view camera is above the fiber tips.

With our earlier Test Stand Zero (TS0), we demonstrated a spiral reset procedure that mitigates hysteresis in the actuators. Following a spiral reset, we obtain precision of 10 μm rms for a subsequent movement to the corners of our range of motion, regardless of where the fiber was located before the move. Once the movement is complete, the actuator creeps. By watching this creep for 200 s, we can predict where the fiber will be 1800 s later with precision 10 μm rms. In operation, we are confident that we will be able to measure the positions of the fibers for a few minutes before the spectrometer exposure begins. Our existing positioners will be able to maintain their fibers with 10 μm rms precision during a half-hour exposure.

We chose steel masts for our Phase I test stands because thin-walled stainless steel tubes are readily available in any length, and easy to machine. Our initial calculation of mast deflection contained an error that underestimated the deflection by a factor of sixty-four, so we were not greatly concerned about the mast deflection. Now that we see the steel masts deflecting by 700 μm, we are looking into using custom-made carbon fiber masts for future positioners. Carbon fiber masts will reduce deflection by a factor of four. We will study the predictability of deflection in our steel masts in order to determine how well we can compensate for deflection in our subsequent carbon fiber masts.

We are building a sixteen-fiber positioner on a 5-mm grid, equipped with all necessary control electronics, power supplies, and amplifiers. In doing so, we will demonstrate that we can pack the required electronics into the space available beneath each fiber. We will mount this positioner on a motorized gimbal and study how the fibers behave as we rotate the positioner about two axes. We will complete a thorough study of its performance by the end of December 2022. We plan to present our gimbal-mounted prototype at the January 2023 AAS meeting in Seattle. We are discussing with Jennifer Marshall at the McDonald Observatory how we might collaborate with her group to install a spectrometer with several hundred fiber positioners on their 2.1-m telescope. In this proposed collaboration, OSI would build the fiber positioner and the fiber view camera, while the McDonald Observatory would build the fiber-coupled spectrometers. Both groups would be funded by the Phase II grant and both groups would work on the installation.

Development Log: Development of the DFPS at OSI starting January 2022.

Base and Service Board (A3043): Combined base and service board for mounting fibers and controllers.

Backplane (A3044): Backplane for connection of service boards.

Fiber Controller (A3045): Logic and amplifiers that generate control signals for actuators.

A Novel Dense Fiber Array for Astronomical Spectroscopy: Application to National Science Foundation (NSF) Small Business Innovation Research (SBIR) agency by Open Source Instruments Inc. Submitted 04-DEC-20, awarded 01-JAN-22, grant number 2111936.

Properties of Piezoelectric Tube Actuators: Study of the movement due to creep in piezo-electric tubes. Guadalupe Duran, Brandeis University, May 2020.

Fiber Positioner Circuits (A2089): Prototype circuits developed at Brandeis University for the DFPS.

News 25-MAR-22: Waltham company helps scientists study the expansion of the universe, article in local business journal.

Direct Positioning of 50,000 Optical Fibers: Poster presented at Snowmass, 2022.