Be aware that what you see is not always what you get :?
I once dismantle a HP mouse with a Logitec chip in it. There is no quatratue signal coming out of these opto couplers at all only a coded signal. So there is no way to "see" the 90° shifted pulses or even to reuse these optocouplers as standard light switch (unless you try to break the Logitec code). The connection pins are +5V; 5VGND and code.
I then dismantle a MS mouse with a Zilog chip. Here again no 90° shifted square pulses. The led is driven by a 5kHz signal out of the Zilog chip so the output of the optocoupler is also a 5kHz signal.
When there is no movement of the ball both signals have an amplitude of 1.5V. Once you turn on the ball both signal start to increase to 5V asynchronously. So instead of 90° shift there is a difference in amplitude to detect L/R movement.
The connection pins are +5V; Signal A and Signal B
Hope this helps, at least you don't have to do these measurements again.