# US 2015/0146751 A1 · High-stability light source system and method

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- Number: US 2015/0146751 A1
- Document ID: US20150146751A1
- Status: Published application
- Kind: A1
- Application number: 14/546,970
- Filed: 2014-11-18
- Published: 2015-05-28
- Earliest claimed: 2013-11-19
- Assignees: USL Technologies, LLC
- Applicants: USL Technologies, LLC
- Inventors: John P. Downing
- Web page: https://patentgrep.com/patent/US20150146751A1

## CPC

- H05B 45/18: using temperature feedback
- A62D 3/00: Processes for making harmful chemical substances harmless or less harmful, by effecting a chemical change in the substances (consuming noxious gases by combustion F23G7/06)
- G01J 3/10: Arrangements of light sources specially adapted for spectrometry or colorimetry
- H01S 5/06804: {by monitoring an external parameter, e.g. temperature}
- H01S 5/0683: by monitoring the optical output parameters
- H05B: ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
- A62D: CHEMICAL MEANS FOR EXTINGUISHING FIRES OR FOR COMBATING OR PROTECTING AGAINST HARMFUL CHEMICAL AGENTS; CHEMICAL MATERIALS FOR USE IN BREATHING APPARATUS
- G01J: MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY (light sources F21, H01J, H01K, H05B; investigating properties of materials by optical means G01N)
- H01S: DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL

## Abstract

A light source system that generates stable optical power over time and temperature in which a feedback control circuit is operative to receive a temperature signal and a sample signal and in response thereto generate a control signal to a driver circuit to maintain a power level of the light output substantially constant over an operative temperature range defined by Tmin and Tmax.

## Claims

### Claim 1

A light source assembly, comprising:
a semiconductor light emitter structured to emit light, the semiconductor light emitter comprising one from a group containing resonant-cavity light-emitting diodes, edge-emitting light-emitting diodes, and super luminescent diodes, wherein the light has a peak wavelength and bandwidth in bands in the group comprising ultraviolet-, visible-, and infrared-emission bands;
a driver circuit having a first input terminal, a second input terminal structured to receive a reference signal, and an output terminal, the driver circuit operative to supply a drive signal on the output terminal to excite light emissions from the semiconductor light emitter;
a lens operative to receive the light and to collimate the light into substantially parallel light rays;
a beamsplitter comprising a plate of optically transparent material having a first surface and a second surface, the beamsplitter positioned to capture and reflect a first portion of the light rays having a first power and transmit a remaining unreflected second portion of the light rays having a second power;
a photodetector circuit having a photodetector with an active area having spatially uniform responsivity over the active area of the photodetector and responsivity that changes less than 0.2% per year and operative to capture substantially all of the reflected first portion of the light rays in the active area and to convert the captured light rays to an electrical sample signal;
a thermally conductive support member comprised of material having thermal conductivity in excess of 220 W/m·° K, the semiconductor light emitter and the photodetector mounted on the thermally conductive support member to maintain them at substantially the same temperatures;
a temperature sensor coupled to the thermally conductive support member and operative to produce a temperature signal representative of the temperature of the semiconductor light emitter and the photodetector; and
a feedback control circuit having a first input terminal coupled to the temperature sensor, a second input terminal coupled to the photodetector circuit, and an output terminal coupled to the first terminal of the driver circuit, the feedback control circuit forming a feedback loop between the photodetector circuit and the first input terminal of the driver circuit and operative to provide a control signal on the output terminal to the first input terminal of the driver circuit.

### Claim 2 (depends on 1)

The light source assembly of claim 1, wherein the driver circuit is operative to maintain the control signal substantially equal to the reference signal.

### Claim 3 (depends on 2)

The light source assembly of claim 2, wherein the driver circuit is structured to adjust the drive signal to excite more or less light emission by the semiconductor light emitter to maintain the power of the second portion of the light rays substantially constant over an operating temperature range defined by Tmin and Tmax.

### Claim 4 (depends on 3)

The light source assembly of claim 3, wherein the photodetector has an operational a characteristic S(T), wherein the responsivity of the photodetector varies with temperature in accordance with dS/dT=dS/dT+β·(dS/dλ).

### Claim 5 (depends on 2)

The light source assembly of claim 2, wherein the feedback control circuit is operative to compute a correction factor responsive to the temperature signal to compensate for a net temperature error and to generate the control signal in proportion to the temperature signal.

### Claim 6 (depends on 5)

The light source assembly of claim 5 wherein the feedback control circuit has an analog-to-digital converter coupled to the first input terminal of the feedback control circuit, a microprocessor coupled to the analog-to-digital converter, a nonvolatile memory coupled to the microprocessor, and a digital-to-analog converter coupled to the microprocessor and to the output terminal of the feedback control circuit.

### Claim 7 (depends on 5)

The light source assembly of claim 5, wherein the feedback control circuit has a second input terminal, first and second analog-to-digital converters in which each analog-to-digital converter is coupled to a respective input terminal of the feedback control circuit, a microprocessor having two input terminals and an output terminal, each input terminal of the microprocessor coupled to a respective one of the analog-to-digital converters, a nonvolatile memory coupled to the microprocessor, and a digital-to-analog converter coupled between the output terminal of the microprocessor and to the output terminal of the feedback control circuit, the first and second analog-to-digital converters operative to provide simultaneous digitization d of the respective temperature and sample signals received on the first and second input terminals of the feedback control circuit.

### Claim 8 (depends on 3)

The light source assembly of claim 3, wherein the feedback control circuit is operative to compute a correction factor in response to the temperature signal and to apply the correction factor to the sample signal to compensate for control errors.

### Claim 9 (depends on 2)

The light source assembly of claim 2, wherein the beamsplitter includes a plate of optically transparent material having interference coatings disposed on first and second surfaces to increase reflectance of the beamsplitter and signal-to-noise ratio of the output light power and increase the light power of the second portion.

### Claim 10 (depends on 1)

The light source assembly of claim 1, wherein the semiconductor light emitter comprises a semiconductor, polarization-locked, single-mode laser operative to emit a beam of coherent polarized light, the light having a polarization ratio of at least 80.

### Claim 11 (depends on 10)

The light source assembly of claim 10, wherein the beamsplitter comprises a glass-wedge beamsplitter having first and second surfaces that meet at an acute angle and having a polarization ratio less than about 1.2.

### Claim 12 (depends on 11)

The light source assembly of claim 11, wherein the driver circuit is operative to maintain the control signal substantially equal to the reference signal.

### Claim 13 (depends on 11)

The light source assembly of claim 11, wherein the driver circuit is operative to excite the laser to emit more or less light so as to maintain the power of the second portion of light substantially constant over an operating temperature range defined by Tmin and Tmax;

### Claim 14 (depends on 11)

The light source assembly of claim 11, wherein the photodetector circuit has a characteristic S(T), wherein the responsivity of the photodetector circuit varies with temperature in accordance with dS/dT=dS/dT+β·(dS/dλ).

### Claim 15 (depends on 1)

The light source assembly of claim 1, wherein the photodetector circuit comprises a resistor structured to couple the semiconductor light emitter to ground potential and operative to provide a sample signal proportional to a forward current through the light emitter.

### Claim 16 (depends on 1)

The light source assembly of claim 1 comprising a reference control circuit having an input operative to receive the temperature signal and to output the reference signal to the second input terminal of the driver circuit in response to the temperature signal.

### Claim 17

A method for calibrating a feedback control circuit and the reference control circuit in a light source assembly, the method comprising:
determining from component test data whether interpolation or numerical solution of a function will be used to minimize a net control error in operation of the light source;
providing a characterization of a variation in peak wavelength with temperature in the light source and forward current through a semiconductor light emitter within a production lot;
providing a computer programmed to control the calibrating of the feedback control circuit and the reference control circuit, including providing a temperature signal and a correction factor;
providing a program enabling a microprocessor in the feedback control circuit to digitize and store a calibration temperature signal and a correction factor;
providing a program enabling the microprocessor in the feedback control circuit to do a numerical iteration to determine an optimal correction factor for each calibration temperature;
providing a means for temporarily bonding a thermally conductive support member to a temperature control apparatus capable of controlling the temperature of the thermally conductive member to an accuracy of 0.1° C. and having a stability of ±0.01° C.;
providing a NIST-traceable photodetector or spectrophotometer for monitoring output light power of the light source in absolute units;
setting the temperature control apparatus to a reference temperature, either Tmin or Tmax, and allowing the temperature signal to stabilize;
setting the output light power of the light source to an absolute reference level P0;
recording the absolute reference level P0 in the computer;
signaling the microprocessor to store a first calibration temperature and the corresponding correction factor;
setting the temperature-control apparatus to the next calibration temperature and allowing the temperature signal to stabilize;
signaling the microprocessor to determine by numerical iteration the value of the correction factor that minimizes a difference between the output optical power and the absolute reference level P0;
recording the temperature signal and the correction factor in memory; and
setting the temperature-control apparatus to the next calibration temperature and allowing the temperature signal to stabilize.

### Claim 18 (depends on 17)

The method of claim 17, comprising repeating the last four steps until the operating temperature range has been calibrated;
sorting calibration data in a table of ascending or descending numerical order, the calibration data comprising temperature signals and corresponding correction factors representing an operating temperature range;
performing a regression analysis to develop a function for determining the correction factor from an arbitrary value of the temperature signal and calculate a coefficient of determination r2; and
programming the function in the microprocessor for operational calculations if r2 is greater than or equal to 0.9999, otherwise programming the table of temperature signals and corresponding correction factors in the microprocessor as a basis for interpolating correction factors from arbitrary temperature values representing the operating temperature range.

### Claim 19

A system, comprising:
a support formed of thermally conductive material;
a semiconductor light emitter mounted on the support;
a driver circuit operative to generate a driver signal to the semiconductor light emitter and to receive a reference signal;
a device structured to receive the light and to reflect a first portion of the light and to output a second portion of the light;
a detector circuit mounted on the support and operative to receive the first portion of the light and generate a sample signal in response to reception of the first portion of light;
a temperature sensor disposed on the support and operative to generate a temperature signal; and
a feedback control circuit operative to receive the temperature signal and the sample signal and in response thereto to generate a control signal to the driver circuit to maintain a power level of the second portion of light substantially constant over an operative temperature range defined by Tmin and Tmax.

### Claim 20 (depends on 19)

The system of claim 19, further comprising a lens operative to receive the light from the semiconductor light emitter and collimate the light prior to reception by the device structured to reflect the first portion of the light.

### Claim 21 (depends on 20)

The system of claim 20, wherein the feedback control circuit comprises a microprocessor operative to generate an error signal that is added to the sample signal to form the control signal.

### Claim 22 (depends on 20)

The system of claim 20, wherein the feedback control circuit comprises a microprocessor having first and second inputs, and first and second analog-to-digital converters in which the first analog-to-digital converter is coupled to the first input of the microprocessor and the second analog-to-digital converter is coupled to the second input of the microprocessor, the first analog-to-digital converter operative to receive the sample signal, the second analog-to-digital converter operative to receive the temperature signal, the first and second analog-to-digital converters operative to digitize the sample signal and the temperature signal simultaneously, the microprocessor operative to simultaneously use the sample signal and the temperature signal to generate the control signal and increase or decrease the control signal to compensate for temperature sensitivity of the source.

### Claim 23 (depends on 20)

The system of claim 20, wherein the feedback control circuit comprises a microprocessor and a switch, the switch operative to receive the sample signal and the temperature signal and to alternately control digitization of the sample signal and the temperature signal, the microprocessor operative to use the digitized sample signal and temperature signal and generate the control signal to increase or decrease the control signal and compensate for temperature sensitivity of the light source.

### Claim 24

A method, comprising:
receiving a reference signal and a control signal and generating a driver signal in response to receipt of the reference signal and the control signal;
generating a beam of light from a light source in response to the driver signal;
reflecting a first portion of the beam of light and outputting a second portion of the beam of light;
receiving the first portion of the beam of light at a detector circuit and generating a sample signal in response to reception of the first portion of the light;
sensing a temperature of the light source and the detector circuit and generating a temperature signal; and
receiving the temperature signal and the sample signal in a feedback control circuit and generate the control signal to maintain a power level of the second portion of light substantially constant over an operative temperature of Tmin and Tmax.