Based on combination method, in [1], T.G. Parham et al. used a Fizeau phase-shifting
interferometer to measure the focus variation l to obtain the UFL focal length, and the
experiment showed that a focal length of 30,000mm can be measured with a relative precision
of 0.02%. In [9], Brian DeBoo and Sasian applied Fresnel holographic lens to the system
instead of RL and have obtained the relative error better than 0.02% for the UFL with focal
length of 9,000mm.
As shown in Fig. 1, the measurement precision of combination method is affected not only
by the measurement precision of l, but also by the precision of f
R
′ and d
0
. However, f
R
′ and d
0
must be measured by using a non-contact method because the UFL in laser fusion system is
coated with anti-reflection film.
In the combination measurement shown in Fig. 1, the key to achieving high-precision
measurements of l, f
R
′ and d
0
is to exactly determine the positions of A, B, C and D by using a
non-contact method.
The methods in [1, 9] used a phase-shifting interferometer to identify the positions of
focuses A and B, so the measurements were easily affected by environmental disturbances.
Furthermore, they cannot directly measure f
R
′ or d
0
. So, the measurement environment
requirements are stringent.
Therefore, a new laser differential confocal ultra-long focal length measurement meathod
(LDCFM) that utilizes self-calibration of f
R
′ and d
0
is proposed in this paper. Furthermore, a
laser differential confocal ultra-long focal length measurement system (LDCFS) for UFL with
large aperture is developed by using LDCFM, which could achieve precise measurements of l,
f
R
′, and d
0
in the same system.
Compared with the existing UFL focal-length measurement methods, the proposed method
has higher measurement precision, better stability and stronger tolerance of environmental
interference.
2. LDCFM principle
As shown in Fig. 2, the LDCFM uses the property that the focus of LDCFS precisely
corresponds to the null points Q
A
and Q
B
of the differential confocal axial intensity curves I
A
and I
B
, to precisely measure the RL focal length f
R
′ by identifying the exact positions of RL
focus and last surface, to measure the lens space d
0
between RL and UFL by determining the
last surface of RL and the vertex of UFL last surface, and to measure the focus variation l in
position of LDCFS with and without the measured UFL, and thereby calculating the UFL focal
length f
T
′ by the above measurements of f
R
′, d
0
and l.
Fig. 2. LDCFM principle. MO is the microscope objective, PH is the pinhole, L
B
is the beam
splitter, L
C
is the collimating lens, UFL is the ultra-long focal length lens, RL is the reference
lens, L
R
is the reflector, DMI is the distance cmeasuring instrument, BS is the beam splitter, M is
the offset of the Detectors from the focus of Lc.
Received 6 May 2015; revised 11 Jun 2015; accepted 14 Jun 2015; published 24 Jun 2015
29 Jun 2015 | Vol. 23, No. 13 | DOI:10.1364/OE.23.017379 | OPTICS EXPRESS 17381