1. Adam Moreau1,2, Jenna Bergevin1, Dr. R. Jason Jones1
1University of Arizona, Tucson AZ ; 2University of San Diego, San Diego, CA
Abstract
A piezoelectric-actuated mirror with greater than 380kHz servo bandwidth was developed, and the principal factors that lead to
low frequency mechanical resonances were examined. In order to keep the gain cavity length of an ultrafast laser uniform, a small
piezoelectric transducer (PZT) is attached to the backside of one of the mirrors. A major difficulty encountered when working with
PZT-actuated mirrors is their very limited bandwidths, often < 20-40kHz. A number of variables were considered for the design of
the mount structure, such as the size of the mount head, the material used on the surface and the core of the mount, the type
and amount of adhesive applied, the methods of application, the mass of the mirror, and fabrication techniques. Out of those
analyzed, the two dominant factors, which most affect the frequency of the first resonance, were the methods for applying the
adhesive, and the thickness of the adhesive used. When adhering the PZT to the mirror and mount head a large adhesive mound
must be depressed, pushing excess material to the sides thus eliminating irregularities within the adhesive itself, and ensuring that
a thin uniform layer is applied. With this addressed, secondary and tertiary factors have been shown to be the presence of a
damping core to prevent longitudinal resonance modes, and the Young’s Modulus of the outer material respectively. The optimal
mount design for a piezoelectric-actuated mirror was the 0.25ӯ Tungsten-Carbide filled design, with an outer material of Brass.
Results
Jenna Bergevin1,2
Dr. R Jason Jones1
Research in Optics (RiO)
Grant # 1460723
Dr. John Koshel
Melissa Sarmiento Ayala, M.Ed.
Mode Locking
Figure 1.
This figure depicts a Fabry-Pérot cavity in which a laser pulse is fed into the
cavity containing some type of gain medium, such as a solid-state crystal or
semiconductor diode. A portion of the beam comes out of the cavity and is
coupled to the output, and a portion reflects back into the gain medium.
The outcome is such that an ultra short pulse train will be produced, and
when the pulses are equally spaced the laser is said to be mode locked.
The optimal mount design for a piezoelectric-actuated mirror is the 0.25ӯ
Tungsten-Carbide cored filled design made of Brass. In all designs the
adhesive wax layer must be thin, as this is a principle factor in determining
the first resonances. Our design utilizes a rigid damping core, and tapered
nose to minimize longitudinal, and drumhead resonance modes respectively.
Due to higher resonances of Brass, compared with Copper or Aluminum, it is
an optimal material for the outer shell. The objective of this research was to
design and fabricate a mount with ≥ 100kHz servo bandwidth. This has been
achieved, and the highest servo bandwidth yet measured by this group is
392kHz.
[1] Briles et al. Optics Express. 18: 9739‐9746 (2010)
[2] Millo et al. Applied Optics. Vol. 53, No. 32 : 7761-7772 (2014)
[3] Li et al. Rev. Sci. Instrum., Vol. 66, No. 1 : 215-221 (1995)
[4] Chadi et al. Rev. Sci. Instrum., Vol. 84, 056112 (2013)
Mechanical Resonance Damping for Piezoelectric-Actuated Mirror Mounts
Project Objective
To investigate the properties of piezoelectric-actuated mirror
mounts which cause low frequency mechanical resonances,
and to design and fabricate a mount with ≥ 100kHz bandwidth
Four Mount Designs were Investigated
• Mounts used, as shown in Fig. 4
Far Left: Cylindrical Control
Center Left: Solid Center of Mass Design
Center Right: Carbide 0.125” Filled Center of Mass Design
Far Right: Carbide 0.25” Filled Design – Adapted from [1]
• Cylindrical mount was used as a control when studying factors such as
PZT/Mirror Location, Mount Material, or Wax Thickness
• Center of Mass Designs were implemented to suppress deflection mode
resonances due to the center of mass of the mount being far from the
attachment point of the mirror mount
• Filled Designs add a damping material to the core of the mount to
suppress longitudinal mode resonances
• Tungsten-Carbide was used for a filling material due to strong damping
properties and ease of fabrication. Increased rigidity reduces the
likelihood of air gaps forming inside the mount during fabrication
Figure 4.
The four designs tested, shown here with Copper outside and Tungsten-
Carbide core.
Front of mounts on bottom and back of mounts are shown on the top.
• A Michelson Interferometer was constructed such that one arm went to a
stationary mirror, and the second arm length was varied by the PZT-actuated
mirror
• As shown in Fig. 3, the PZT wires were run to a BNC Cable. This could either be
attached to the output of a PZT Driver or the output of the Network Analyzer for
single frequency or wide spectrum analysis respectively
• When taking a wide spectrum (1Hz-500KHz) the photodiode was connected to
the input of the Network Analyzer, and each frequency sweep was averaged to
increase the signal to noise ratio
Figure 3.
Image of Interferometer used
for measurement of
mechanical resonances
Red line shows beam path,
with PZT-actuated mirror
labeled A, stationary mirror
labeled B, and photodiode, C
A
B
C
Application for Mode Locking
Figure 2.
Frequency Combs (2005 Nobel Prize), shown above, are a direct result of
mode locking. Frequency Combs applications have far ranging promises
from precision spectroscopy in the XUV and VUV range, to development of
an optical clock. By increasing the useable bandwidth of the piezoelectric-
actuated mirror, the stability of the comb is greatly improved, allowing for
further developments
Time Domain
Dependence upon Wax Thickness
Figure 5.
The first resonance is highly dependent upon the wax layer
thickness. Using Crystal Bond 509 adhesive wax, a pillar was
constructed and the frequency response measured as the
height was varied. When modeled as a damped spring the
resonant frequency falls off as h-1/2 as shown.
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Thickness of Wax [cm]
0
20000
40000
60000
80000
100000
Frequency[Hz]
Wax Thickness
Data Points
Fit Curve
90% Bounds
0 50000 100000 150000 200000 250000 300000 350000 400000 450000 500000
Frequency [Hz]
0
0.5
1
1.5
2
ArbatraryUnits
Amplitude
Brass
Copper
0 50000 100000 150000 200000 250000 300000 350000 400000 450000 500000
Frequency [Hz]
Phase[Degrees/360] Phase
Brass
Copper
0.25ӯ Filled Mount Design
Figure 8.
Longitudinal resonance modes were further
damped by increasing the size of the Tungsten-
Carbide core, and shrinking the overall length of
the mount. During fabrication, the high rigidity of
Tungsten-Carbide, in contrast with other
damping materials such as lead [1], allows for
precise machining such that no air gaps can form
as the heated outer shell cools around it. The
resulting frequency responses are highly stable in
both amplitude and phase, and approach the
free resonance of the bare PZT.
Copper - Filled: 380kHz
Brass– Filled: 392kHz
Center of Mass Mount
Figure 7.
The traces above correspond to the Center of Mass Mount
Design (Fig. 4b,c). No significant change was found for
Brass (margin of error), and a slight decrease occurs with
Copper, indicating that deflection modes are not critical
factors for these materials. It is hypothesized that this
effect may be due to the higher Young’s Modulus of Brass
and Copper than Aluminum. The filled mount did however
improve performance by damping longitudinal modes.
Aluminum: 102kHz
Brass: 104kHz
Copper - Solid: 89kHz Copper – Filled: 142kHz
Material Comparison
Figure 6.
The traces above are from 1”Ø by 1” cylindrical mounts
(Fig. 4a) using Crystal Bond 509 with mounts of Aluminum,
Brass and Copper. Averaged data was taken three times for
each material resulting in the following resonance peaks:
Aluminum: 84kHz
Brass: 106kHz
Copper: 96kHz
0 50000 100000 150000 200000 250000 300000 350000 400000 450000 500000
Frequency [Hz]
0
0.5
1
1.5
2
2.5
3
ArbatraryUnits
Amplitude
Aluminum
Brass
Copper - Solid
Copper - Filled
0 50000 100000 150000 200000 250000 300000 350000 400000 450000 500000
Frequency [Hz]
Phase[Degrees/360]
Phase
Aluminum
Brass
Copper - Solid
Copper - Filled
0 50000 100000 150000 200000 250000 300000 350000 400000 450000 500000
Frequency [Hz]
0
0.5
1
1.5
2
2.5
ArbatraryUnits
Amplitude
Aluminum
Brass
Copper
0 50000 100000 150000 200000 250000 300000 350000 400000 450000 500000
Frequency [Hz]
Phase[Degrees/360]
Phase
Aluminum
Brass
Copper
Additional Factors Investigated
• 1” mirror mount brand and/or direction of mount was not found to
affect the frequency at which resonances occur
• No measurable difference was shown between two adhesives, Crystal
Bond 509 and Mounting Wax 80
• Increased mirror mass lead to a lowered first mechanical resonance
[2][4]
• Placement of the PZT/Mirror off the center of the mount was shown
to negatively impact the resonant frequency by up to 10kHz
Our design for mechanical resonance damping has proven effective,
however fabrication can introduce minor variations in the first resonant
frequency (±4kHz). Additional adhesive application methods should be
investigated and standardized, as this has been identified a source of minor
fluctuations between tests. There are many types of adhesive waxes beyond
those covered by the scope of this study. These may prove to have
advantageous properties such as having an increased young’s modulus [3],
or ability to applied in even thinner layers [2]. In addition to fabrication
improvements, mounts are being implemented into an active Ti:Sapphire
Servo loop to study performance in active systems.
Support
Conclusion Future Work
Resonance Measurement Technique
Mount Design
Background
References