So far we have established that a Michelson interferometer with a baseline of 500 km operating at an input power of 1 W could in principle measure strains of the order h ∼ 10−21, typical of realistic astrophysical sources of gravitational waves. The problem with this design of course is that it requires one to build an enormous instrument. This problem is solved in LIGO and Virgo instruments byfolding the interferometer arms. Since it is the optical path length rather than the physical path length that matters for length sensing, one can fold the interferometer arms so that the distance traveled by any single photon is typically many times the physical length of the instrument.
One way of folding the instrument arms is with a Fabry-Perot optical cavity, as depicted in Fig. 2.2. A Fabry-Perot cavity consists of two highly reflective mirrors, aligned such that an input
Figure 2.2: Fabry-Perot optical cavity.A Fabry-Perot optical cavity consists of two highly reflective mirrors facing each other. The use of such optical cavities in LIGO instruments in place of the usual Michelson end mirrors increases the optical path length from the input beam to the output beam at the photodiode, allowing for high sensitivity strain sensing with a baseline over 100 times smaller than would otherwise be required. This cartoon drastically over-simplifies the optical properties of Fabry-Perot cavities used in real LIGO instruments. The laser beam is depicted here for simplicity and clarity by geometric rays incident at an angle to flat cavity mirrors. In real applications, the laser beam has finite width, and great care must be taken to align the mirrors (which are curved) such that the laser beam is resonant in the cavity.
beam incurs multiple reflections before exiting the cavity. At every encounter with a mirror, some part of the light exits the cavity, and each of these transmitted beams interfere with each other. The light that remains in the cavity continues to bounce around, picking up a phase shift
∆φ=2πL
λ (2.12)
for each one-way trip down the cavity. The summed beam effectively reflected at the input mirror contains contributions from fields with any integral number of phase shifts 2n∆φ, and consequently has an intensity which is highly sensitive to changes in the length of the cavity. We will see here that by replacing the end mirrors in a Michelson interferometer with Fabry-Perot cavities, we can achieve the desired strain sensitivity with only kilometer-scale arm lengths.
Consider a Fabry-Perot cavity consisting of two highly reflective mirrors with reflection coeffi- cientsr1andr2and transmission coefficientst1andt2, corresponding to reflection and transmission from the left at the left and right mirrors, respectively, as shown in Fig. 2.2. A beam of light is incident normally from the left upon the first mirror (the rays are shown at an angle in Fig. 2.2 for clarity). The initial beam is partially reflected and partially transmitted. If the initial electric field is E~0, then the reflected field2is related to the input field byE~1refl=r1E~0and a fieldE~1=t1E~0enters the cavity. The transmitted field crosses the cavity and encounters the second mirror, picking up a
2For consistency with Sec. 2.1, the reflection coefficient at the initial mirror, which was the only mirror in the Michelson design, must be positive. Within the cavity, the reflection coefficients are then forced to be negative.
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phase shift of ∆φ= 2πL/λ. Again, part of the beam is reflected and part is transmitted. We obtain a transmitted field at the second mirrorE~trans1 =t2E~1ei∆φ=t1t2E~0ei∆φ. This process continuesad infinitum. The reflected beam crosses the cavity, picking up an additional phase ∆φ. Part of this beam emerges from the left mirror, now withE~2refl =−t21r2E~0e2i∆φ, and the rest is reflected back into the cavity. Continuing in this way, we can compute the superimposed reflected and transmitted beams from the cavity via the infinite series:
E~refl1 = r1E~0
E~refl2 = −t21r2E~0ei2∆φ E~refl3 = −t21r1r22E~0ei4∆φ
...
E~n≥2refl = −t21r2e2i∆φE~0 r1r2ei2∆φn−2
... +
E~refl = r1E~0−t21r2e2i∆φE~0
X∞ n=0
r1r2e2i∆φn
(2.13)
= E~0
r1− t21r2e2i∆φ 1−r1r2e2i∆φ
. (2.14)
Note that reflection at the initial mirror from the right incurs an additional minus sign. In a similar manner, we can compute the electric field transmitted by the Fabry-Perot cavity, which results in
E~trans=E~0
t1t2ei∆φ 1−r1r2e2i∆φ
. (2.15)
The transmitted power is then Ptrans
Pin = t21t22
1−2r1r2cos(2∆φ) +r21r22 (2.16) and the reflected power is given byPrefl=Pin−Ptrans.
Now imagine that we have replaced the two end mirrors of a Michelson interferometer with two Fabry-Perot cavities (see the boxed out region in Fig. 2.3). What is the intensity of the resulting field at the output port? We neglect the Michelson length degree of freedom, that is, the distance between the beam splitter and the initial mirror of the Fabry-Perot cavity. To simplify the calculations, we will assume that the initial mirrors are identical and the end mirrors are perfectly reflecting. That is,
Figure 2.3: Advanced LIGO optical layout. We show the basic optical subsystems of the aLIGO instrument design. The Michelson-Fabry-Perot model we have considered in this chapter, highlighted by the dashed box, forms only one of the many optical cavities. As mentioned in the caption for Fig. 2.2, our geometric optics approximation to the electric fields in the optical cavities is a gross oversimplification of the physics of these devices. In particular, the mirrors in a Fabry-Perot cavity are typically curved and the lowest resonant mode is not a standing plane wave. Fabry-Perot cavities have not one but infinitely many resonant modes.
A typical laser beam has a profile similar to the fundamental mode of the cavity, but contains contributions which may excite higher order modes of the Fabry-Perot. The input and output mode cleaners remove these higher order modes.
we taker1X =r1Y =randr2X=r2Y = 1. Assuming lossless mirrors, the transmission coefficients are determined by conservation of energyt2= 1−r2. We then allow an input beam to pass through the beam splitter. As before, half of the incident light is reflected at the beam splitter and the other half is transmitted. These two beams are then incident upon their respective downstream Fabry- Perot cavities. Using Eqn. 2.14, we treat each Fabry-Perot cavity as a single mirror with effective reflection coefficient
ri=r−(1−r2)e4πiLi/λ
1−re4πiLi/λ , (2.17)
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where i ∈ {X, Y} and Li is the length of the Fabry-Perot cavity in the i-th arm. Ignoring the Michelson degrees of freedom (the distance between the beam splitter and the initial mirrors) and applying Eqn. 2.1, we have for the electric field observed at the photodetector
E~ = E~0
2 (rX−rY) (2.18)
and the power is given by
P =|E~|2=|E~0|2
4 (|rX|2+|rY|2−2Re{rXr∗Y}). (2.19) Compare this result to Eqn. 2.2, which we derived for a simple Michelson interferometer. Note that since we have assumed lossless, perfectly reflecting end mirrors, themagnitudes of the now complex reflection coefficientsrX andrY are each unity, and we obtain
P =|E~|2=|E~0|2
2 (1−Re{rXrY∗}). (2.20)
The sensitivity to length changes in the cavities comes (as before) in the cross terms in which the relative phase shifts between the two beams of light lead to interference. In Fig. 2.4, we show the dependence of the power observed at the output port on the relative length changes for a simple Michelson, and comparing this setup to a Michelson coupled to Fabry-Perot light storage cavities with perfectly reflecting end mirrors.