A novel photonic analog-to-digital converter (ADC) based on asynchronous delta-sigma modulation technique (ADSM) has been investigated and demonstrated. The ADSM architecture utilizes photonic leaky integrators, inverted bistable quantizer and positive corrective feedback suitable for non-interferometric optical implementation. The principles of the proposed 2nd -order ADSM are modeled and simulated mathematically. A prototype fiber-optic ADSM is constructed, producing acceptable NRZ type binary output for frequencies in the MHz range. For an input of 2 MHz and sampling rate of 10.8 MHz, the prototype achieves SNR of 39.8 dB, SFDR of 21.2 dB and ENOB of 6 in the 5.4 MHz bandwidth of interest.