Focus sensing apparatus using electrical AGC to enhance differential focus error signal

A method of detecting the focus condition of radiation energy in an optical system comprising the steps of:

• focussing said radiation upon an object;
• separating at least a portion of radiation reflected by said object into transmitted and reflected beams;
• detecting the intensities of said transmitted and reflected beams;
• adjusting said detected intensities to compensate for disturbances in said optical system;
• generating a first signal indicative of the detected intensity, as adjusted, of said transmitted beams;
• generating a second signal indicative of the detected intensity, as adjusted, of the reflected beam;
• and
• generating a differential focus error signal based in part on said first and second signals.

A focus sensing apparatus comprising:

• a source of radiation energy;
• a lens which focuses radiation from said source upon an object to be reflected therefrom;
• a beam separation module which receives at least a portion of radiation reflected by said object and separates said portion into transmitted and reflected beams;
• photodetectors which receive said transmitted and reflected beams, said photodetectors detecting the intensities of said transmitted and reflected beams and generating output signals representative of said intensities;
• and
• circuitry responsive to said output signals of said photodetectors, wherein said circuitry adjusts said output signals to compensate for disturbances in said optical system.

A focus sensing apparatus comprising:

• a source of radiation energy;
• a lens which focuses radiation from said source upon an object to be reflected therefrom;
• a beam separation module which receives at least a portion of radiation reflected by said object and separates said portion into transmitted and reflected beams;
• a first photodetector channel which receives said transmitted beam and generates a plurality of output signals;
• a second photodetector channel which receives said reflected beam and generates a plurality of output signals;
• a first summing amplifier which receives said plurality of output signals from said first photodetector channel and generates an output signal indicating the focus condition of said transmitted beam;
• a second summing amplifier which receives said plurality of output signals from said first photodetector channel and generates an output signal indicating the intensity of said transmitted beam;
• a third summing amplifier which receives the output of said second photodetector channel and generates an output signal indicating the focus condition of said reflected beam;
• a fourth summing amplifier which receives the output of said second photodetector channel and generates an output signal indicating the intensity of said reflected beam;
• a first divider circuit which divides said output signal from said first summing amplifier by said output signal from said second summing amplifier, thereby generating a first compensated focus error signal indicating the compensated focus condition of said transmitted beam;
• a second divider circuit which divides said output signal from said third summing amplifier by said output signal from said fourth summing amplifier, thereby generating a second compensated focus error signal indicating the compensated focus condition of said reflected beam;
• and
• a fifth summing amplifier which subtracts said first compensated focus error signal from said second compensated focus error signal to obtain a compensated differential focus error signal indicating the focus condition of said radiation reflected by said object.

A focus sensing apparatus comprising:

• a source of radiation energy;
• an objective lens for focusing said radiation energy upon an object;
• a beam separation module for separating at least a portion of the light reflected by said object into transmitted and reflected beams;
• a beam splitter for directing at least a portion of said reflected energy into said module;
• a detector arrangement for receiving said transmitted beam and said reflected beam and for generating in response thereto a first signal indicative of a characteristic of said transmitted beam and a second signal indicative of a characteristic of said reflected beam;
• and
• a circuit receiving both said first and second signals from said detector arrangement, for generating a focus error signal based on both said first and second signals.

A method of detecting the focus condition of radiation energy in an optical system comprising the steps of:

• focussing said radiation upon an object;
• separating at least a portion of radiation reflected by said object into transmitted and reflected beams;
• detecting the intensity and distribution of said transmitted and reflected beams;
• generating a transmitted focus error signal based on the detected intensity of said transmitted beam;
• adjusting said transmitted focus error signal to compensate for disturbances in said optical system;
• generating a reflected focus error signal based on the detected intensity of said reflected beam;
• adjusting said transmitted focus error signal to compensate for disturbances in said optical system;
• and
• generating a final focus error signal based on said adjusted transmitted focus error signal and on said adjusted received focus error signal.