Abstract: Disclosed is an apparatus for verifying data written on an optical disk by simultaneously reading the data during writing and using differential detection to verify that the data has been written correctly. The data is written in a fixed-block format defining a symbol having a predetermined number of symbol positions and a predetermined number of holes which may be written in the symbol positions, an equal number of holes in the odd positions and in the even positions. In this regard, the holes written in the odd positions are independently verified from the holes written in the even positions. Immediately after the laser has been operated at write power, it is returned to read power and a signal associated with the hole associated power of the reflected beam is sampled. This signal is compared to the signal power generated at the previous even or odd position. The highest signal is retained and compared to the next even or odd position's signal.