What is the importance of a high modulation depth?

Question

I understand that the modulation depth of a  sinusoidally modulated signal can be defined as the modulation amplitude divided by the mean value, as explained here.
But why would one wish for a high modulation depth in an experiment? What advantages does it bring?
Some articles state that they have achieved a high modulation depth of 90%, but isn't what matter that amplitude of the signal or its "shape"/frequency?

hyportnex · Accepted Answer

The higher the modulation index is in a double sideband AM system (DSB) the higher is the SNR when detected; this is true for both coherent and non-coherent (envelope) detection methods.
Write the DSB-AM signal as $$x(t)=A_c(1+kappa m(t)) rm{sin} (omega_c T) tag{1}label{1}$$ where $0 < kappa le 1$ is the modulation index, $-1 le m(t) le 1$ the modulation (information bearing signal) and $omega_c$ is the carrier frequency whose amplitude is $A_c$. Assume that the signal is received in white normal noise of intensity $mathcal N_0$ then for coherent detection the signal is detected with $$(SNR)_D=2(SNR)_T frac{kappa^2 langle m^2 rangle}{1+{kappa^2 langle m^2 rangle}} tag{2}label{2}$$
Here $langle m^2 rangle$ is the variance of the modulation, and  $(SNR)_T=frac{P_T}{N_T}$ with $N_T=2mathcal N_0 W$ and $P_T=frac{1}{2}A_c^2(1+{kappa^2 langle m^2 rangle})$ being the received RF noise and RF transmit powers, resp.
 For non-coherent detection the received SNR is $$(SNR)_D=frac{2(SNR)_T}{1+frac{2}{(SNR)_T}} frac{kappa^2 langle m^2 rangle}{(1+{kappa^2 langle m^2 rangle})^2} tag{3}label{3}$$
 As you can see from $eqref{2}$ and $eqref{3}$ both coherent and non-coherent detection SNR are monotonically increasing function of the modulation index $kappa$, higher the index $kappa$ the higher the $SNR$ is.
 An all-around excellent book to read on this (chapter 8) and  also on many other subjects is
[1] Ziemer and Tranter: PRINCIPLES OF COMMUNICATIONS: Systems, Modulation, and Noise, Wiley 7th ed.
A general note of caution: in practice, the true performance of DSB-AM with high modulation index is quite sensitive to both transmitter and receiver nonlinearities that are always present and will inevitably degrade the theoretical SNR discussed here.

The Photon · Answer

In the context of intensity-modulated optical communications systems, a high modulation depth is preferred because it implies high efficiency, where efficiency is defined in terms of bits of data transferred per joule of energy.
Only the modulation amplitude contributes a useful signal. The power that is present at the minimum of the signal doesn't carry any information. It increases the energy sent per bit without improving the signal-to-noise ratio of the system.
Practically, though, the goal is usually to achieve a modulation depth that is "high enough" rather than to maximize it, because beyond a certain point there are diminishing returns for additional improvements in modulation depth.
You would see a similar benefit (i.e. better signal discrimination with less optical power) if you were using a high-modulation-depth modulator, for example, to control the exposure of a sample in a fluorescence experiment (you wouldn't want to continue exposing the sample to pump light during the emission phase of the experiment).
And, as presented in your linked article, a high modulation-depth absorber is also essential to constructing a high-quality mode-locked laser.

What is the importance of a high modulation depth?

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