An apparatus includes a wavelength swept light source to generate a measurement beam, a scanner to move the measurement beam on a sample, a measurement device to measure the sample, and a safety device to stop the light source based on sweeping status of the light source.

Methods, systems and devices for estimating a parameter of interest in a borehole. The method may include generating information from an optical displacement device relating to relative motion between two or more reflective surfaces thereof that is indicative of the parameter of interest; and preventing changes in the information resulting from changes at the optical displacement device in at least one of i) temperature, or ii) pressure, by compensating for the changes. Compensating may include adjusting at least one light source generating an electromagnetic beam at least partly received by the optical displacement device responsive to information relating to a control optical displacement device at the optical displacement device. Compensating may include using an optical displacement device and configuring the optical displacement device such that a difference between a first variable gap and a second variable gap is substantially zero while the apparatus is subject to nominal conditions.

An incoming laser source beam is projected to a glass disc having a frosted upper surface. A refracted beam and a reflected beam are produced from the laser source beam. The refracted beam is reflected back to and through the glass disc to combine with the reflected beam to form an interference pattern on the target.

Methods, systems and devices for estimating a parameter of interest in a borehole. The method may include generating information from an optical displacement device relating to relative motion between two or more reflective surfaces thereof that is indicative of the parameter of interest; and preventing changes in the information resulting from changes at the optical displacement device in at least one of i) temperature, or ii) pressure, by compensating for the changes. Compensating may include adjusting at least one light source generating an electromagnetic beam at least partly received by the optical displacement device responsive to information relating to a control optical displacement device at the optical displacement device. Compensating may include using an optical displacement device and configuring the optical displacement device such that a difference between a first variable gap and a second variable gap is substantially zero while the apparatus is subject to nominal conditions.

An apparatus includes a wavelength swept light source to generate a measurement beam, a scanner to move the measurement beam on a sample, a measurement device to measure the sample, and a safety device to stop the light source based on sweeping status of the light source.