Laser Noise Couplings in RSE
5.5 Transmission of Light
5.5.2 The Carrier
Eq. (5.15) can be expanded to lowest order by first deriving appropriate expression for the arm cavity average and differential reflectivity. This is done in Appendix A, and given in Eq. (A.20) and Eq. (A.26).
(5.26) (5.27)
The following definitions are used, where it's been assumed that the end mirror re- flectivity is unity, and so the losses and transmittance of the ETM must be included in the losses of the ITM in the calculation of ritm·
9 a = - - -2 1-Titm
C 1-Titm W c = - -
2l arm
.,;r::;;;;
'ntm Tc 0
=
Titm-1-Titm
(5.28) (5.29) (5.30) The explicit frequency dependence will clutter the formulas, so a convention will be adopted that puts the frequency dependence into the superscript. A
"+"
will indicate a positive frequency component, "-" a negative one, and "0" for a DC component. In the case that a variable is used for both the carrier and RF sidebands, two superscripts will be used, the first indicating the carrier, upper, or lower RF sidebands by a c,+,
or - respectively, followed by the superscript which indicates the frequency of the noise component. Subscripts will typically be used to indicate degrees of freedom, or c for arm cavity. The one exception to this rule is in the t's of the transmission functions, and E's of the fields, where the frequency dependence isin the subscript.
Appendix B.2 derives the relevant equation for the carrier from Eq. (5.15). Terms are kept to first order in the phase offsets. The DC carrier term is given by
while the frequency dependent term is
These use Eq. (5.26), Eq. (5.27), as well as the following definitions
(5.31)
(5.32)
(5.33)
(5.34)
(5.35)
(5.36)
These last few equations define the frequency dependent interferometer gain, where the frequency dependence is carried in H±. The shape of H± has two zeros at the arm cavity pole We and two poles which are the arm/power and arm/signal coupled cavity poles. A nice derivation of the poles and zeros of coupled cavities is done by Rahkmanov. [62]
As could be expected, the transmission at DC is strictly dependent on imperfec- tions, either in differential mode offsets or arm mismatch, as seen by the terms in brackets of Eq. (5.31). This is nearly true for the transmission of the noise sidebands on the carrier as well, with the exception of the coupling via the asymmetry by ( w6 /c) as seen in Eq. (5.32).
DC Carrier Term
It's useful to make a comparison of the sensitivity of the RSE interferometer with the initial LIGO I interferometer, in order to get a sense for how much the specifications might change. In the initial LIGO I configuration, with no signal mirror, it was noted that the largest source of noise was due typically to noise on the RF sidebands which beat against the carrier mismatch contrast defect. [59] The level of contrast can be compared in RSE to LIGO I by examining the ratio
ltprmAbstsem9fJol tprmAbs9f_7o,LIGO I
(5.37)
These terms in the ratio are the interferometer-dependent gains for the amplitude of the mismatch carrier defect. The ratio then assumes the same input power and the same level of or~. The LIGO I interferometer gain is given by
9ijo,LIGO cO I
=
1+
r A 0 prm bsTc1 (5.38)
Figure 5. 7 plots this ratio using the same optical parameters (except for the addition of the signal mirror, of course). Figure 5. 7 shows that the signal tuned interferometer
101 ~~~~~~~~~~~.~ ... ............. T ..... ~ .. ~ .. 7 .. ...~.~-~.~-~~~~~~~~~~~~ . ········. ·······. . . . . ····. . ·. ·. ·. · . . . . . . . . . . . . . . . . .
········· ······· ····. ·············.······ .· ...... : ... .
. . . . . . . : . . . ~ : : : : : : : : : : : : :: : : : : : : : . . . . . . . . . . . .. . . :;::::::::::::
········ ......... ···. ····· .... ................. . ... .
········ ...... · ............. .
10_, ' - - - ' - - ---'-- - -- L _ _ _ __L _ _ _ _L_ _ _ _ _ l _ _ _ _ ..J
0 0.5 1.5 2 2.5 3 3.5
Detuning phase (radians)
Figure 5. 7: Ratio of the RSE DC carrier gain to that of the LIGO I configuration, as a function of detuning phase. The optical parameters are Titm
=
3%, Tprm=
1%, and Tsem=
10%.suppresses the contrast defect by nearly a factor of ten for most all detunings. The
source of the suppression has two factors. First is simply the amplitude transmissivity of the signal mirror, which in this case is 0.32. Second is the suppression due to the fact that the signal cavity is actually anti-resonant at the carrier frequency. The cavity phase itself is at a resonance condition for broadband RSE, but the fact that the arms are over-coupled adds a phase of 1r upon reflection, hence making the cavity anti-resonant. The transmissivity of an anti-resonant cavity is roughly 1/2 that of the transmissivity of the mirrors involved. The combination of these two factors suggests a transmissivity which is roughly 0.16 times that of LIGO I, which is what the figure shows. This degrades somewhat as the signal cavity is detuned away from the RSE point, although the mismatch carrier defect suppression remains fairly good for most detunings. At the dual-recycling point, however, it's seen that this defect is actually enhanced. This was a result also discovered by the modeling efforts of Bochner.[29]
Carrier Noise Sidebands
The comparison between an RSE interferometer and the LIGO I configuration for the transmission of the frequency dependent part of the carrier transmission can be made as well. Eq. (5.32) gives the equation for the transmissivity of the RSE interferometer for the carrier noise sidebands. Clearly, the noise sidebands couple through the imperfections via the gain of tprmAbstsem9fJaH±. The only difference between this and the previous section is the frequency dependence contained in the interferometer gain term, H±. The overall DC gain suppression factor for RSE will be the same. For LIGO I, without the signal cavity, the second pole and the second zero in Eq. (5.34) don't exist.
H/icoi=
( 1 ±
i~)
( 1 ±
i~)
Wee (5.39)LIGO I has a pole at the coupled cavity pole, which is typically about 1 Hz. The zero is the arm cavity zero, around 100Hz. Thus, the frequency response for carrier noise sidebands in LIGO I rolls off at 1 Hz, then flattens out at 100 Hz. In the bandwidth
of interest, then, the noise sidebands are about a factor of 100 smaller than the DC term.
The RSE response is given m Eq. (5.34), and has two zeros at 100 Hz, the arm/power coupled cavity pole at 1 Hz, and the arm/signal cavity pole, which is characteristic of the frequency response of the arm/signal coupled cavity system.
This is typically much larger than the arm cavity pole frequency in broadband. Even for a detuned interferometer, this will typically be larger than the arm cavity pole frequency, since the signals of interest are in the several hundred Hz range. The response is then expected to roll off at 1 Hz, as before, but at the arm cavity pole frequency, the two zeros cause the response to increase. The details of the signal cavity determine the overall coupling in the bandwidth of interest. In the broadband RSE case, this will be a simple pole at some fairly high frequency. For a detuned interferometer, the pole will be complex, and a somewhat more complicated response is expected.
An example using the same parameters as in the previous section is displayed in Figure 5.8, for a broadband RSE interferometer, and one detuned by </Jdt