Pre-Phase A Report - Lisa - Nasa

58 Chapter 3 Experiment Description
Shot noise limit. Single frequency laser light of significant power from a lightweight
reliable system is best provided by a monolithic Nd:YAG laser pumped by laser diodes.
With this type of laser up to 2 W of light is currently achievable. In order to obtain high
reliability from such a system in a space environment it is reasonable to derate the laser
by a factor of two and use only P = 1 W of output power. The laser system is described
in detail in Section 3.1.5. Using this laser and taking into account the overall optical
efficiency we find that shot noise limits the minimum detectable change 1 to
δx =11×10 −12
3
λ
1064 nm
2 0.3W
ɛPo
1
2
L
5×10 9 m
30 cm
D
2
m/ √ Hz , (3.2)
where L is the arm length, λ the wavelength and Po the power of the laser, the efficiency
of the optical chain is ɛ, andD is the diameter of the transmitting and receiving optics.
Thus with an optical chain with a realistic efficiency of ɛ =0.3 and a mirror of diameter
of D ≈ 30 cm the target shot-noise performance can be achieved.
3.1.5 Laser system
Introduction. The laser system to be used in the LISA mission is a diode-laser-pumped
monolithic miniature Nd:YAG ring laser which can generate a continuous diffractionlimited
infra-red beam at 1064 µm ofupto2W.
Diode-pumped solid-state lasers, operating in a single transverse mode, are well known
as compact, reliable and highly efficient sources of stable radiation. In the case of the
NPRO (Non Planar Ring Oscillator), TEM 00 mode operation is achieved by focussing the
diode-laser beam into the crystal (see below). When the beamwidth of the diode laser
radiation is smaller than the diameter of the TEM 00 mode in the absorption length of
the pump radiation, the laser is forced to operate in a single transverse mode.
Homogeneously broadened solid-state lasers oscillate on several longitudinal modes even
at low output power because of the spatial hole-burning effect. To enforce single-frequency
operation, resonator internal elements can be applied. However, the additional intracavity
elements strongly reduce the efficiency and stability of the laser system. The monolithic
Nd:YAG ring laser enables single-frequency operation at high output power without intracavity
elements. Unidirectional and hence single-frequency oscillation is enforced by
an intrinsic optical diode.
The optical beam path in the crystal is determined by three total reflections and one
reflection at the negatively curved front surface. The front surface is dielectrically coated,
reflecting about 97 % of the 1064 nm laser radiation and highly transmitting the pump
radiation at 808 nm (see Figure 3.4). The high frequency stability required for the LISA
mission can only be achieved because of the high intrinsic stability of the NPRO (see
Section 3.1.6). This stability results from the monolithic and compact design of the
resonator and from the outstanding properties of host material YAG (Yttrium Aluminum
Garnet Y3Al5O12). Low CTE (7×10−6 K−1 ) and low temperature dependence of the
1The convention above and throughout this paper is that a ‘tilde’ over a quantity indicates that it is
a linear spectral density, e.g. δx is the linear spectral density of δx.
3-3-1999 9:33 Corrected version 2.08