Photoacoustic joulemeter utilizing beam deflection technique
Inventors
Elliott, William Rowe • Glickman, Randolph D. • Barsalou, Norman • Maswadi, Saher M.
Assignees
University of Texas System • US Department of Navy • Henry M Jackson Foundation for Advancedment of Military Medicine Inc
Publication Number
US-8711342-B2
Publication Date
2014-04-29
Expiration Date
2028-08-11
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Abstract
A joulemeter is capable of non-destructively measuring multiple characteristics of a laser beam. The joulemeter comprises a series of parallel probe beams, which are directed though a transparent media adjacent to an absorbing media that the tested beams pass through. Arrays of optical sensors or a chirp sensor are used to intercept and measure deflections the probe beams. A control unit renders measurements on selected properties of the laser.
Core Innovation
The invention is a joulemeter for non-destructively measuring multiple characteristics of laser or electromagnetic radiation beams in real time over a wide dynamic range. It employs a combination of the photoacoustic principle, where light energy is converted into heat producing acoustic or pressure waves in a propagating medium, and the photothermal deflection technique (PDT), where energy as a propagating wave is measured by deflection of probe beams. The joulemeter utilizes multiple parallel probe beams directed through an absorbing medium or an adjacent transparent medium. These probe beams are deflected by refractive index gradients caused by pressure waves generated from the absorption of laser energy, and sensors detect the deflections for analysis.
The joulemeter addresses known problems with existing laser energy measurement devices that require beam interruption, slow response times, cooling periods, or destruction of the beam. Traditional thermal detectors such as calorimeters, thermopiles, and pyroelectric detectors, as well as photonic detectors like photoconductive, photovoltaic, and photoemissive devices, suffer from slow response, wavelength dependency, low damage thresholds, or destructiveness. The invention provides a non-contact, highly sensitive, and fast responsive system that avoids the need for absorbing the entire beam, thus enabling real-time and repeatable measurements with minimal sensor damage.
The invention may include multiple absorbing media with different optical absorption coefficients selectable based on the laser wavelength to optimize measurement accuracy and prevent damage. Probe beams are measured by sensor arrays or chirp sensors which do not obstruct the laser beam. A control unit processes sensor data and outputs laser beam parameters such as spot size, beam diameter, beam profile, total energy, pulse energy, and power. The invention is adaptable to different operational modes including pulsed and continuous wave lasers and can be configured for two-dimensional profile measurements using an orthogonal grid of probe beams.
Claims Coverage
The claims encompass inventive features of a joulemeter device and methods for non-destructively measuring laser or electromagnetic radiation beams using probe beams and selectable absorbing media.
Non-destructive beam measurement using absorbing media and probe beams
The device includes multiple absorbing media aligned with the radiation source such that the beam passes through at least one absorbing medium. It incorporates at least one light source generating multiple probe beams directed through the absorbing media or an adjacent medium, sensors positioned to receive and measure probe beams without obstructing the radiation beam, and a control unit that selects absorbing media based on expected wavelength and converts sensor signals into beam characteristic measurements.
Selection and mechanical positioning of absorbing media based on wavelength
A control unit selects an appropriate absorbing medium considering the expected wavelength of the radiation and commands mechanical means, such as motors or rotors, to align the selected medium in the beam path.
Use of parallel or substantially parallel probe beams and specific sensor types
Probe beams run substantially parallel. Sensors include arrays or matrices of photodiodes, chirp photodiodes, CCD, CMOS sensors, or quad photodetectors to measure deflections caused by acoustic waves without obstructing the beam.
Capability to measure multiple laser properties
The control unit processes sensor data to provide measurements of pulse energy, beam diameter, energy density, beam power, beam profiling, and spot size.
Modulation of continuous wave lasers using disk-shaped absorbing media
For measuring continuous wave lasers, a disk-shaped absorbing medium comprising at least two regions with different absorption coefficients rotates at a known frequency to modulate the beam, enabling measurement by probe beam deflection.
Method steps for measuring pulse and continuous wave laser beam properties
Methods involve selecting absorbing media based on laser power and absorption coefficient, positioning absorbing media in the beam path, directing probe beams through adjacent media, measuring probe beam deflections with sensors that do not obstruct the beam, and rendering measurements of selected beam properties.
Method for controlling laser beam characteristics using probe beam deflections
The method includes positioning rotating disk-shaped absorbing media with different absorption coefficients in the beam path, measuring probe beam deflections, rendering beam characteristic measurements, and modulating control parameters of the laser or radiation beam to achieve desired beam characteristics.
The claims collectively cover a non-destructive joulemeter employing selectable absorbing media and parallel probe beams measured by various sensor arrays to generate real-time data on laser beam characteristics, including methods to measure and control continuous wave and pulsed laser beams with modulated absorbing media.
Stated Advantages
Provides accurate, real-time, non-destructive measurements of laser radiation over a wide dynamic range.
Enables real-time measurement of operational laser parameters including beam profile, spot size, pulse energy, and power.
Suitable for different operational modes such as pulsed or continuous wave lasers.
Allows measurement without substantial absorption or splitting of the laser beam, minimizing sensor damage.
Low cost to build, maintain, and operate compared to existing devices.
Enables automatic selection and calibration of absorbing media based on laser wavelength and power.
Documented Applications
Monitoring and controlling lasers used in medical diagnosis or treatment, including lithotripsy, photo-coagulation, photo-ablative therapies, surgical applications, and in vivo quantitation of pharmaceutical concentrations.
Use in combination with pharmacological agents targeted to specific cells or tissues to control laser impact.
Monitoring and control of high energy lasers or electromagnetic beams in industrial applications such as welding, machining, pharmaceutical and food quality control.
Applications in military directed energy weapons (DEWs) for monitoring, ranging, tracking, targeting, damaging, or disabling various targets.
Laboratory applications including laser-based fluorescence microscopy and detection of chemical, biological, or nuclear agents.
Use in National Board of Standards calibration for high energy lasers producing 300 J or more per pulse.
Measuring drug distribution in a patient’s eye using photoacoustic signals generated by laser interaction with pharmacological agents.
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