Certain difficulties related to an employment of current methods, involving the use of a laser as radiation source, in optical absorption experiments can be overcome with the aid of a new approach. This approach was used in an experiment in which a He-Ne laser was employed for monitoring the concentration of iodine vapor in air. Two optical cavities are formed with three mirrors by using one mirror surface to form the interface. Thus, by the addition of one high reflectivity mirror and the shared use of one laser mirror, a secondary cavity to the laser can be created which behaves in a fashion similar to a multipass absorption cell. The surface reflection at the outer surface of the laser mirror is the only shortcoming, but antireflection coating will eliminate the greater part of this reflection.