5.2 Cavity Design
5.2.3 Nanopositioning System
There are many ways to conceive of positioning a thin film between two FP mirrors (at least as many late nights for an obsessive fourth-year graduate student, I assure you). To constrain ourselves, and to manage uncertainties about the alignment challenge, we adopted the following pragmatic approach:
we would build the nanopositioning system around the traditional cavity with minimal change to its design, and in a manner which would allow us to insert and remove, in situ, the membrane from the cavity, thereby enabling us to perform upgrades to the chip and chip holder in a “plug-and-play”
fashion. In return, we were willing to sacrifice some compactness, vibrational stability, vacuum compatibility, and cavity finesse.
The design that we ultimately settled on is pictured in Figure 5.3. It represents a compromise between flexibility, stability, and some insistence on integrating standard, commercially available optomechanics and nanopositioning hardware (as opposed to a custom nanopositioning system).
It also represents a natural extension of the standardized setup in our lab for the construction of short, high-finesse cavities, in which one of the mirrors — now replaced by the membrane — is manipulated in the arm of a 5-axis stage designed for fiber alignment. Generally speaking, a commercially available nanopositioning system with our coarse and fine resolution requirements consists of either a piezo/microstepper motor-actuated translation stage or else one of the now standard slip-stick nanopositioning systems pioneered by Attocube [74]. Although less elegant, we chose the former approach because it is considerably less expensive and more extensible. From
picomotor actuators
translation stage (Newport 561)
membrane chip holder optical cavity
kinematic mount (Newport Suprema)
kinematic mount membrane chip holder membrane shaker pzt membrane chip input mirror substrate input mirror v-block input mirror shear pzt
"common mode" shear pzt stainless steel platform exit mirror
Figure 5.3: Schematic of the MIM apparatus including nanopositioning system.
among the growing family of motorized actuators available, we chose the New Focus Picomotor because of its good reputation with our NIST colleagues. For the translation stage, we chose the 561 fiber positioning stage from Newport, as this stage was both compact and had been used with success for cavity construction and in a project to pull fiber tapers for our group’s toriod- based CQED experiment [75]. To incorporate tip/tilt, we fixed the stiffest available kinematic mount from Newport (Suprema SN100) to the top of the translation stage. The resulting fine resolution for each axis is set by the nominal 30 nm stepsize of the Picomotor: {x, y, z,tip,tilt} ≈ {30 nm,30 nm,30 nm,0.8µrad,0.8µrad}. The coarse range is set by the range of the stage and kinematic mount: {12.5 mm,12.5 nm,12.5 nm, >10 deg, >10 deg}.
The final ingredient that needed to be developed was a jig to adapt the membrane — which must be suspended between the cavity mirrors — to the kinematic mount without sacrificing optical access to the cavity, and without diminishing the mechanical properties of the membrane. This was achieved by judicious lathe-working of a 1” cylindrical steel blank, as shown in Figure 5.4 (a recent upgrade uses Macor and an additional long-range piezo, as shown alongside). A brief digression into details of the Picomotors and the “membrane holder” is provided below.
5.2.3.1 Picomotors
All three axes of the translation stage as well as the tip/tilt axes of the kinematic mount are fitted with standard open-loop, high-vacuum-compatible New Focus Picomotors (model 8302V).
The operating principle of the Picomotor is common to all resonant piezo motors — a piezo-electric element at the interface of two frictionally contacted surfaces is vibrated in such as way as to shimmy the two surfaces away from each other. In this case the two surfaces are a screw and its shaft separated by a piezo-electric sleeve, and the action of the piezo vibration is a small advance of the screw inside of its thread. By this “squiggle motor” mechanism, Picomotor actuators are capable of advancing a ball-tip screw over a range of centimeters in microsteps of approximate (but repeatable) magnitude∼30 nm. Aside from having a smaller step size, their advantages over standard DC servo motors or stepper motor actuators are true “set it and forget it” operation (de-powering the Picomotor does not result in any micromotion), small backlash (typically 1–2 microsteps), compactness, vacuum compatibility, and economic pricing/availability. Their main disadvantage is a load-dependent step size, which greatly diminishes their utility as bi-directional scanning devices. Closed-loop Picomotors are available which compensate for this deficiency, but are less compact and a great deal more expensive.
Our Picomotors have performed well for us as unidirectional scanners in the axial (z) direction and as coarse positioners in the transverse (x, y) and tip/tilt directions. Indeed, contrary to our original intention, we have relied heavily on the use of a Picomotor to translate the membrane in fine steps along the cavity axis. In doing so, we have become well acquainted with the load hysteresis
of these devices. We have found, in particular, that the step size for our stage varies between 20–26 nm and is typically ∼20% different between the backward and forward directions. A HeNe fringe formed between the membrane chip and the back surface of one of the cavity mirrors (see Figure 5.7) has been used to verify the unidirectional step repeatability of the Picomotor over several fringes.
For an example of this measurement, see Figure 6.4 in Chapter 5.
5.2.3.2 Membrane Holder
The concept for the membrane holder pictured in Figures 5.3 and 5.4 and derives from considerable trial and error in the process of learning to mount chips containing high-Q suspended films, as discussed in Section 4.7. As discussed in that section, we found that rigidly attaching the chip to a surface tended to reduce (in some cases significantly) the mechanical quality of the membrane, and that this reduction could be minimized by making the attachment points small and as far from membrane as as possible. Aside from minimizing contact area with the chip, the membrane holder needed to provide optical access, convenient integration into the nanopositioning system, and a piezo for fine positioning or resonant driving of the piezo. Two iterations of the design are shown in Figure 5.4. The currently used jig consists of a 1” x 1” stainless steel cylinder machined to slide over the cavity mirrors as pictured. The rear of the 1” cylinder secures into the kinematic mount. The front end is fitted with a 1-mm-thick monolithic piezoelectric plate (Ferroperm Pz27), which has been slotted on a diamond band saw to provide three-point contact with a 5 mm membrane chip. The chip is secured with three small dabs of commercial-grade Krazy Glue (low viscosity cyanoacrylic adhesives allow for rigid bonds with small contact area), which we found to reduce the quality factor of a 50 nm×500µm×500µm membrane by ∼ 30% (see Section 4.7.1). The piezo plate has an unloaded resonance frequency of∼1 MHz and a travel range of ∼1 nm/(10 V). We unfortunately lost the ground electrical lead on this jig while opening the chamber, rendering it useless for long- range displacement or for applying a calibrated high-frequency dither to the membrane position (in the latter case due to capacitive pickup). However, it is still useful for ring-down excitation.
A second-generation design, which has very recently been integrated, replaces the stainless steel with Macor for better electrical isolation and incorporates an additional ultra-compact, long-range piezoelectric ring stack from Noliac (20 nm/V, model CMAR05).
Figure 5.4: Schematic of the membrane chip holder.