Download Thoughts on R+D for Gamma Gamma Optical System Josef Frisch

Thoughts on R+D for Gamma Gamma Optical System
Josef Frisch
Major Technical issues:
1. Cavity stability
a. Longitudinal: several requirements need to be met simultaneously
i. Fringe width ~1 nanometer (not as bad a gravity wave detector,
but still difficult).
ii. Modelocked line matching / dispersion matching: Has been
demonstrated for low power, CW modelocked systems
iii. Mode shape matching – narrow waist (near unstable cavity) only
allows for a small number (possibly 1) of usable fringes. The
calculated tolerance is approximately 500 nanometers.
b. Transverse
i. Near-unstable cavity requires high stability on cavity mirror angle.
Angular stability has not been calculated, but is expected to be less
than a microradian.
ii. The cavity instability may also place tight requirements on the
pointing stability o the input laser.
c. Mirror surface figure
i. For simple 2 mirror concentric resonators, mirror surface figure is
critical (at the lambda / 1000 level). For the telescope cavity this
has not been calculated, but presumably requires adaptive optics
for mode shape correction.
ii. The cavity contains multiple elements. It must be determined if a
single adaptive optic is sufficient, or if multiple optics, with
independent feedback loops are required.
2. Optical Damage:
a. The damage threshold for this pulse structure is not well known
b. The system must operate for ~10^9 pulse trains. Multi-shot damage must
be tested. Additionally, the cavity and optics design must not allow small
laser spots to be formed on any optical element due to minor
misalignments of the system.
3. Pulsed Power Effects
a. Photon pressure: The ~30MW circulating power (1 millisecond) produces
a force of approximately 0.1 Newton. If we assume a mass of ~300kg, The
optical pressure will accelerate the mirror over a distance of 1nm in
approximately 2.5 milliseconds. This suggests that the photon pressure
may be significant.
b. Thermal distortion: Some of the circulating power will be absorbed in the
mirror coatings. During a (millisecond) pulse each mirror is exposed to
approximately 15 KiloJoules of optical energy, approximately 1 Joule /
cm^2. This energy will heat the coatings and the mirror substrate. If we
assume an averaged thermal expansion of 1e-6/C, and a power absorption
of 1e-4, the distortion is 1/10 nanometer – close enough to our tolerance to
require a detailed calculation. Average power distortion must be
calculated, but is almost certainly significant.
c. Thermal induced coating changes. The coating index of refraction, and
therefore the effective reflecting surface of the mirror will be temperature
dependant. This should be estimated.
4. Feedback:
a. A suitable measurement of the cavity mode properties to allow feedback is
required. The length and steering feedback can probably be controlled by
injecting optical sidebands into the cavity (as is done with LIGO), but
analysis of the cavity line structure is required to determine if this scheme
works for a near-unstable cavity with its near-degenerate line frequencies.
b. The feedback actuators must have low delay and high bandwidth in order
to correct for the expected high frequency disturbances from the
millisecond laser pulses.
c. Since power cannot be coupled into the cavity unless it is aligned, it is
likely that a low power CW (or CW mode locked) alignment laser is
required. The feedback transition between the low power and high power
systems at the start of the pulse may be complex.
d. The feedback algorithm may be very complex, with both optical and
mechanical time-constants. Often problems than can “in principal” be
solved with feedback are in practice intractable.
5. Hardware
a. The 1.2M diameter, high damage threshold, low loss mirrors need to be
investigated. It is not clear that these can be manufactured in existing
coating chambers. Note that high damage threshold coatings are often
produced by a different process than low loss coatings.
b. The large and possibly fast adaptive optic needs to be studied, to
understand its mechanical bandwidth.
c. Other large optical components, such as the telescopes to image the large
cavity mode onto a detector need to be designed. While these do not
appear to be pushing the state of the art, they may be expensive to develop.
It is likely, for example, that imaging the IP phase of the cavity mode will
require a 1.2M diameter telescope, optimized to correct the distortion
caused by transmission through the off axis focusing mirrors.
6. Drive laser:
a. Although the cavity reduces the required laser power by a factor of
approximately 500, a ~300 Watt average, 300KW macropulse peak,
approximately 1 millisecond (300 Joule ) laser is still required. This is still
very large laser.
b. The laser output pulse structure is very inconvenient. The combination of
short pulses and long trains is possible, as demonstrated by the TTF gun
laser, but has only been demonstrated at ~ 1 Watt average powers. Note
that the TTF gun laser itself is the result of a multi-year R+D project.
c. The high laser peak powers (~10 GW) typically require the use of chirped
pulse amplification. It is not clear that compression gratings can operate at
the required average power levels.
d. This laser must have an M^2 of near 1 (or even higher powers are
required). Typically M^2 becomes more difficult as the average laser
power increases. The combination of average power and M^2 is near the
state of the art even for CW lasers.
Suggested R+D.
Tolerance calculations: Linear cavity analysis should be sufficient to determine the
position and angle stability requirements for each of the optical cavity elements.
Estimates from heat dissipation and photon pressure can be used to estimate the
mirror distortions due to the optical power. This could possibly be done by the time of
the Snowmass meeting.
Feedback Design: The cavity system will require multiple feedback loops, probably
including: Cavity length feedback for resonance, Dispersion feedback, element
steering feedback, adaptive mirror shape feedback. It may be possible to allow the
adaptive mirror system to perform all of the above. A basic feedback scheme, which
sensors, which actuators, can be developed from a linear model of the cavity behavior
and line structure.
Full simulation: If the optically induced distortions exceed the tolerances derived
above, the cavity behavior will be sufficiently complex that a full simulation should
be performed. There are 3 parts to the simulation code:
Optical propagation in the cavity, including diffraction, for arbitrary (small)
mirror distortions. Codes for this (for example GLAD) exist. Calculations
may need to be performed for each frequency, and then the results combined.
Mirror distortion including dynamics for the thermal load, and photon
pressure. This can probably be modeled using ANSYS or a similar finite
element code.
Feedback: The use of feedback elements to control the cavity mode.
The 3 parts of the code will need to be integrated. The optical time constant for the
cavity is on the order of 30 microseconds, It is not clear if this is sufficiently fast to
treat the mechanical motion as quasi-static for the optical analysis.
The simulation is a large project but should have wide applicability to high power
optical signals. Code for this analysis may already exist.
Cavity test 1: Build a simple linear cavity sized to fit on an optical table (~2M). After
initial testing, the cavity can be mounted in a vacuum chamber. Mirror radii chosen to
be near-unstable, to provide angular tolerances comparable to those in the gammagamma cavity. It will not be possible to match all of the parameters of the full scale
cavity, but a near-unstable cavity will provide a simple test bed for feedbacks. Note
that the instability of the cavity can be modified by changing the cavity length.
A possible testing plan is
1. Low power CW laser, length and pointing feedbacks only, no adaptive optics).
2. Low power CW / modelocked laser with length, pointing, and dispersion
feedbacks.
3. Low power CW / modelocked laser, add adaptive optic mirror(s) for mode
shape control.
4. Pulsed laser: Develop feedback loops to deal with the large optical dynamic
range.
Cavity test 2: Build a telescopic cavity at 1/3 scale. Cavity is designed for low power
at 350nm wavelength. The optical behavior of the full scale cavity should scale with
the wavelength, and most conventional optics and detectors will work at this
wavelength. A frequency tripled mode-locked laser can be used as a source. High Q
mirrors are available at this wavelength if damage threshold is not an issue.
Note that this test is only valuable if the small scale cavity is substantially less
expensive than a full scale cavity at the correct frequency.
Cavity test 3:Construct a full scale cavity, and high power laser. Cavity should be
tested in a lab, without the mechanical constraints of the detector interface.
Damage Testing: Optical damage testing (on small optics) with realistic pulses should
be conducted in parallel with the above tests.
Laser: A large laser development program is obviously required. The LLNL
“Mercury” laser represents a good starting point, but does not meet the transverse
mode shape or pulse structure requirements.
Notes:
If time is more critical than money, the first cavity, and the simulations could be
developed simultaneously.
Cavity simulations may indicate that some feature of the cavity is the most
problematical. In that case, the cavity 1 test can be optimized to model that feature.
Overall comments: (mostly negative)
This project has a complexity and difficulty which may exceed that of LIGO (itself
the result of over 20 years of R+D, and > $100M ).
The transverse mode requirements for the drive laser are far beyond the state of the art
for that power and for complex picoseconds pulsed lasers. The M^2 of a laser can be
thought of as similar to the emittance of an accelerator. In both cases the difficulties
increase with power (or current), and are not typically amenable to simple solutions.
The M^2 of existing laser systems is considered an important parameter, and has
already been pushed very hard. Improvements may not be practical.
While the cavity does not have the length stability requirements of LIGO, it is in
every other respect far more difficult. The high power, pulsed operation, and nearunstable design introduce a wide range of technical issues.
The components of this system are likely to be very expensive and very long lead time
(1.2 M, high precision, high damage threshold, fused silica optics), and, in a high
power system, can easily be destroyed during testing. This is likely to result in a very
expensive and slow development project.