As a point of interest, all optical encoders and scales of this type, TTL included, start off with an optically detected signal that is sine wave in nature, in a TTL Incremental quadrature encoder type, the sine wave is squared up and converted to TTL and/or Differential output.

In these Heidenhain and similar scales or encoders, the arc tangent function is used to produce a high resolution absolute encoder count from the sine/cosine signal.

As suggested, you could use an op amp to amplify this signal and square it up and produce a TTL quadrature signal.

Nosmo.