Low f/# optics
for laser-wire

Requirements

Constraints

Methodology

Designs

f#/2 design (532 nm)

First lessons from the f#/2 lens

f#/1 design (532 nm)

f#/1 design (266 nm)

Requirements
for a micrometre resolution
laser-wire

To create a ~1 micrometre spot size with 532nm light, one need to focus the focus the light with low f# optics near the limit set by diffraction (f#=focal length/aperture)

Very few commercial lens available with f#/1 or f#/2,
none suits our needs

Diffraction limited systems are challenging

An aspheric element is likely to be needed

The spot size must remain constant over a “scanning” range.

Constraints
on the optics design (1/3)

Constraints from the laser

High power: 1J/200ps = 1 GW@532nm
Ghost focus formed by single or double bounces of the light on the surfaces of the lens may be an issue.

Limited choice of glass: must not absorb too much power, only glasses compatible with high power lasers (Fused silica, BK7,...) are acceptable.

Must be coated to limit internal reflections

Bandwidth: Nd:YAG laser
=> bandwidth required <1nm

Constraints
on the optics design (2/3)

Constraints from the accelerator:

Clearance: Must leave enough clearance for the beam during tuning	=> Back focal length>15mm (20mm in fact)

Radiations: Glass must be resistant to the radiation level experienced near the accelerator
At the ATF, a BK7 lens has survived this year's run without damage.

Vacuum: Need a window to allow the transition from air to vacuum; deformations of the window may be an issue at high power
=> fused silica window, 12.7mm thick, Indium seal.

Constraints from the scanning: Must be able to move the spot's vertical position over ~1-2 mm to search and “scan” the e- beam.

Constraints
on the optics design (3/3)

Mechanical constraints
on the lens

Methodology for
the lens design (1/2)

Design done with a dedicated software (Zemax)

Design start with basic layout and constraints:
3 elements (aspheric, spheric and window)

A “merit function” is defined to define how to evaluate a given design:

What is the spot size variation acceptable over the scanning range?

Is a smaller spot size better than a smaller f#/?

Methodology for
the lens design (2/2)

An initial optimization is done with a “genetic” algorithm and a basic merit function.

Once a correct design is found, it is optimized with a more advanced merit function (but much slower to calculate).

A Damped Least Square (DLS) algorithm is used to do the final optimization of the lens design with a final merit function.

f#/2 lens design@532nm

Design started by R. Bingham

Focal length: 56mm

Back focal length: 24mm

Aperture f#/2

All elements in fused silica

No primary ghosts, one secondary ghost

Expected sigma
~2 micrometres

f#/2 lens design:
Ghost issues

This design has a second order ghost

Ghost is weakly focused

Assuming less than 0.5% reflection on each surface, risks of damages are limited
(calculations indicate that it should not be a problem at our operating power)

f#/2 lens design@532nm:
Tuning and spot adjustment

The spot size can be adjusted from 30 um to 2um by varying the laser beam aperture & divergence
=> important for tuning

The spot can be translated along the laser axis by varying the gap between the window and the 2 other elements.

The spot can be moved in a plane parallel to the window by translating the 2 other elements in that plane.

f#/2 lens design@532nm:
First lessons

Lens has been manufactured and is ready for testing.

Testing will be done this summer.

Due to budget constraints we did split the construction of different elements with different supplier. This has resulted in significant delays later.

Over-specification of the mount also caused significant delays (and loss of a supplier).

F/#1 lens @532nm (1/3):
Candidate design 1

Aspheric doublet

Wavefront optimized

F/# 1.05

Ray at 0.3 degree produce a bigger spot!

Performance estimator: 0.09

F/#1 lens (2/3):
Candidate design 2

Aspheric doublet

Spot size optimized

F/# 1.37

The size of the spot size varies less with the angle

Perf. estimator: 0.07

Worse F/# but better estimator due to the large angle rays

F/#1 lens (3/3):
Candidate design 3

Spheric triplet

Wavefront optimised

F/# 1.62

Very good homogeneity (<10%)

Probably cheaper than an asphere

Estimator: 1.44

f#/1 lens design@266nm

In the U.V., the design is much more difficult.

Work is still very preliminary

Some candidate designs found so far with spot below 1 micrometre but none is satisfactory (inconsistent scanning).

Zemax seems to be less adapted to work in the UV

More designs under study...

Outlook

The design of low f# optics for laser-wire applications is challenging.

A f#/2 lens has been produced and will be tested this summer.

Several f#/1 designs @532nm have been studied, optimisation still to be done.

First f#/1 designs @266nm under study: difficult but some solutions seem to exist.

2006-07-01T20:48:41