If the arms of the interferometer have different lengths, however, the exact cancellation of the laser frequency fluctuations, say , will no longer take place at the photodetector. In fact, the larger the difference between the two arms, the larger will be the magnitude of the laser frequency fluctuations affecting the detector response. If and are the lengths of the two arms, it is easy to see that the amount of laser relative frequency fluctuations remaining in the response is equal to (units in which the speed of light )[5*, 13, 35], Eq. (1*) implies the following expression for the amplitude of the Fourier components of the uncanceled laser frequency fluctuations (an over-imposed tilde denotes the operation of Fourier transform):
A first attempt to solve this problem was presented by Faller et al. [17*, 19, 18], and the scheme proposed there can be understood through Figure 1*. In this idealized model the two beams exiting the two arms are not made to interfere at a common photodetector. Rather, each is made to interfere with the incoming light from the laser at a photodetector, decoupling in this way the phase fluctuations experienced by the two beams in the two arms. Now two Doppler measurements are available in digital form, and the problem now becomes one of identifying an algorithm for digitally canceling the laser frequency fluctuations from a resulting new data combination.
The algorithm they first proposed, and refined subsequently in , required processing the two Doppler measurements, say and , in the Fourier domain. If we denote with , the gravitational-wave signals entering into the Doppler data , , respectively, and with , any other remaining noise affecting and , respectively, then the expressions for the Doppler observables , can be written in the following form:3*) and (4*) it is important to note the characteristic time signature of the random process in the Doppler responses , . The time signature of the noise in , for instance, can be understood by observing that the frequency of the signal received at time contains laser frequency fluctuations transmitted earlier. By subtracting from the frequency of the received signal the frequency of the signal transmitted at time , we also subtract the frequency fluctuations with the net result shown in Eq. (3*).
The algorithm for canceling the laser noise in the Fourier domain suggested in  works as follows. If we take an infinitely long Fourier transform of the data , the resulting expression of in the Fourier domain becomes (see Eq. (3*))
The problem with this procedure is the underlying assumption of being able to take an infinitely long Fourier transform of the data. Even if one neglects the variation in time of the LISA arms, by taking a finite-length Fourier transform of, say, over a time interval , the resulting transfer function of the laser noise into no longer will be equal to . This can be seen by writing the expression of the finite length Fourier transform of in the following way:6*) implies that the finite-length Fourier transform of is equal to the convolution in the Fourier domain of the infinitely long Fourier transform of , , with the Fourier transform of  (i.e., the “Sinc Function” of width ). The key point here is that we can no longer use the transfer function , , for estimating the laser noise fluctuations from one of the measured Doppler data, without retaining residual laser noise into the combination of the two Doppler data , valid in the case of infinite integration time. The amount of residual laser noise remaining in the Fourier-based combination described above, as a function of the integration time and type of “window function” used, was derived in the appendix of [53*]. There it was shown that, in order to suppress the residual laser noise below the LISA sensitivity level identified by secondary noises (such as proof-mass and optical path noises) with the use of the Fourier-based algorithm an integration time of about six months was needed.
A solution to this problem was suggested in [53*], which works entirely in the time-domain. From Eqs. (3*) and (4*) we may notice that, by taking the difference of the two Doppler data , , the frequency fluctuations of the laser now enter into this new data set in the following way:7*) against how they appear in Eqs. (3*) and (4*), we can further make the following observation. If we time-shift the data by the round trip light time in arm 2, , and subtract from it the data after it has been time-shifted by the round trip light time in arm 1, , we obtain the following data set: 8*) from Eq. (7*) we can generate a new data set that does not contain the laser frequency fluctuations ,
In order to gain a better physical understanding of how TDI works, let’s rewrite the above combination in the following form9* [45*].
Equation (10*) shows that is the difference of two sums of relative frequency changes, each corresponding to a specific light path (the continuous and dashed lines in Figure 2*). The continuous line, corresponding to the first square-bracket term in Eq. (10*), represents a light-beam transmitted from spacecraft 1 and made to bounce once at spacecraft 3 and 2 respectively. Since the other beam (dashed line) experiences the same overall delay as the first beam (although by bouncing off spacecraft 2 first and then spacecraft 3) when they are recombined they will cancel the laser phase fluctuations exactly, having both experienced the same total delays (assuming stationary spacecraft). For this reason the combination can be regarded as a synthesized (via TDI) zero-area Sagnac interferometer, with each beam experiencing a delay equal to . In reality, there are only two beams in each arm (one in each direction) and the lines in Figure 2* represent the paths of relative frequency changes rather than paths of distinct light beams.
In the following sections we will further elaborate and generalize TDI to the realistic LISA configuration.