Direct Fiber Positioning System

[04-MAY-22] Our Direct Fiber Positioning System (DFPS) uses only the bending of a short piezo-electric cylinder to displace the tip of an optical fiber by means of a long tube attached to the cylinder. Each fiber positioner consists of a cylindrical, piezo-electric actuator, a long tube for a mast, a polished fiber tip held in a ferrule, and a controller and amplifier circuit to generate the actuator's electrode voltages. The DFPS will provide one fiber positioner per 5 mm × 5 mm square in the image plane of a telescope, thus providing one measurement of red shift per 25 mm2. 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 mrad. The mast is 300 mm long, so the movement of the fiber is ±1.8 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 4.8-mm square. We consider the complications of extending the mast in our development page.

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.
Guide CameraA camera looking down on the fiber tips.
Table Glossary of DFPS Terminology.

The photograph below shows a single prototype fiber positioner. 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 to produce ±1.8 mm translation of the fiber tip. The positioner is soldered onto a 5-mm square footprint on a printed circuit board. The mast is fastened with epoxy to the end of the actuator. An optical fiber terminates in a 2.5-mm diameter white zirconia ferrule at the top of the mast. The optical fiber runs down the center of both mast and actuator and out through a hole in the bottom of the platform. Four ±250-V voltages enter the support platform via coaxial cables to cause the actuator to bend, which in turn moves the tip of the mast in a 3.6-mm square. We shine light into the far end of the fiber and watch the tip of the fiber with a precision camera that looks down from above. The camera allows us to measure the movements of the fiber with 5-μm precision.

Figure: A Three-Fiber Positioner. Each positioner consists of a 40-mm actuator, 300-mm steel mast, and 2.5-mm diameter zirconia ferrule mounted on a base board. The guide camera is above the fiber tips.

Our Phase I work began in January 2022. We constructed our three-fiber positioner and studied the behavior of the actuators and masts. We find that a spiral reset procedure is effective at mitigating the hysteresis of the actuators. Following a spiral reset, we obtain precision of 10 μm for a subsequent movement to the corners of our 3.8 mm ×3.8 mm range of motion, regardless of where the fiber was located before the move. Once the movement is complete, the fiber moves as 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 also. In operation, we are confident that we will be able to illuminate our fibers and measure their actual positions until the spectrometer exposure begins. Our existing positioners will be able to maintain their fibers with 10 μm precision during a half-hour exposure.

Our plan for the remainder 2022 is as follows. We will build 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 in 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 have this prototype operational by the first week in July 2022. We will complete a thorough study of its performance by the last week of August. Starting in May 2022 we will begin calling domestic telescopes and discussing their spectrographic measurement ambitions. We want to find a telescope with an existing fiber-coupled spectrographic back-end to which we can connect our fiber positioner. By the last week of September we will establish a partnership with an existing telescope. The objective of this partership will be to install a 500-fiber positioner in Phase II of our SBIR project. In the second week of October we will submit our Phase II proposal. We will continue work on refining our prototype and planning our Phase II program through the end of 2022. If we stil have funds left in our grant, we will request a no-cost extention to our Phase I program and continue perparations for Phase II until the end of March 2023.

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

Controller Board (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.