Find all injective functions $ f:\mathbb{N} \to \mathbb{N} $ such that $$ f^{f\left(a\right)}\left(b\right)f^{f\left(b\right)}\left(a\right)=\left(f\left(a+b\right)\right)^2 $$ holds for all $ a,b \in \mathbb{N} $. Note that $ f^{k}\left(n\right) $ means $ \underbrace{f(f(\ldots f}_{k}(n) \ldots )) $
1. **Initial Setup and Definitions**:
   We are given the functional equation:
   \[
   f^{f(a)}(b) f^{f(b)}(a) = (f(a+b))^2
   \]
   for all \(a, b \in \mathbb{N}\), where \(f^k(n)\) denotes the \(k\)-th iterate of \(f\) applied to \(n\).

2. **Injectivity and Initial Claims**:
   - **Claim**: \(f(x) \neq 1\) for all \(x\).
     - **Proof**: Suppose \(f(k) = 1\) for some \(k\). Then, using the functional equation with \(a = b = k\):
       \[
       f^{f(k)}(k) f^{f(k)}(k) = (f(2k))^2 \implies f(k) f(k) = (f(2k))^2 \implies 1 = (f(2k))^2
       \]
       This implies \(f(2k) = 1\), which contradicts the injectivity of \(f\) since \(f(k) = f(2k) = 1\). Therefore, \(f(x) \neq 1\) for all \(x\). \(\blacksquare\)

   - **Claim**: \(f(1) = 2\).
     - **Proof**: Suppose \(f(1) \neq 2\). Consider the functional equation with \(a = b = 1\):
       \[
       f^{f(1)}(1) f^{f(1)}(1) = (f(2))^2 \implies (f^{f(1)}(1))^2 = (f(2))^2
       \]
       Since \(f\) is injective, \(f^{f(1)}(1) = f(2)\). If \(f(1) = k\), then \(f^k(1) = f(2)\). If \(k > 2\), this would imply a contradiction as \(f\) would not be injective. Therefore, \(f(1) = 2\). \(\blacksquare\)

3. **Functional Equation Analysis**:
   - Using \(P(a, 1)\):
     \[
     f^{f(a)}(1) f^2(a) = (f(a+1))^2
     \]
   - Using \(P(a, a)\):
     \[
     f^{f(a)}(a) f^{f(a)}(a) = (f(2a))^2 \implies (f^{f(a)}(a))^2 = (f(2a))^2 \implies f^{f(a)}(a) = f(2a)
     \]

4. **Inductive Step**:
   - **Base Case**: We have \(f(1) = 2\) and \(f(2) = 3\).
   - **Inductive Hypothesis**: Assume \(f(n) = n+1\) for all \(n \leq t\).
   - **Inductive Step**: We need to show \(f(t+1) = t+2\).
     - Using \(P(t, 1)\):
       \[
       f^{f(t)}(1) f^2(t) = (f(t+1))^2
       \]
       Since \(f(t) = t+1\), we have:
       \[
       f^{t+1}(1) f^2(t) = (f(t+1))^2
       \]
       Given \(f^2(t) = f(f(t)) = f(t+1)\), we get:
       \[
       f^{t+1}(1) f(t+1) = (f(t+1))^2
       \]
       Since \(f^{t+1}(1) = t+2\), we have:
       \[
       (t+2) f(t+1) = (f(t+1))^2 \implies t+2 = f(t+1)
       \]
       Thus, \(f(t+1) = t+2\).

5. **Conclusion**:
   By induction, we have shown that \(f(n) = n+1\) for all \(n \in \mathbb{N}\).

The final answer is \(\boxed{f(x) = x+1}\).