Wavelength monitored and stabilized source

Methods and apparatus for sampling techniques can constantly monitor a spectral output from a broadband source in order to control a central wavelength of interrogation light supplied by the source for input to a sensor. A first portion of light output from the broadband source passes through a controller module for spectral analysis and referencing to provide measurements that can be used as feedback to actively modify a second portion of the light from the source. This modified second portion thereby controls the central wavelength to ensure accurate determination of sensor response signals received at a receiver.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the invention generally relate to optical sensor systems and, more particularly, to improving wavelength stability in broadband source light used to interrogate optical sensors.

2. Description of the Related Art

Optical sensor systems operate by exposing a portion of an optical waveguide to an environmental condition that modulates a light signal transmitted within the optical waveguide. This modulation alters one or more parameters of the light transmitted within the optical waveguide, such as amplitude, power distribution versus frequency/wavelength, phase, or polarization. Analyzing modulated light emerging from the waveguide enables determining values indicative of the environmental condition. Such systems utilize sensors based on, for example, Bragg gratings or interferometers, to measure a wide variety of parameters, such as strain, displacement, velocity, acceleration, flow, corrosion, chemical composition, temperature or pressure. In one example of an optical sensor system, a fiber optic gyroscope (FOG) enables measuring angular rotation since application of force alters the wavelength of light as it travels through a sensing coil of an optical fiber, thereby producing phase changes from which measurements can be made.

Instabilities in a center wavelength of input light provided by a broadband light source may cause variations in sensor response signals produced upon the interrogating light arriving at the optical sensor. For example, broadband sources producing input light without a stable center wavelength when used with a Bragg grating sensor may cause variations in the reflected response signal emitted by the sensor, resulting in incorrect measurements or undesirable noise. In the FOG, the phase change with acceleration depends on wavelength such that any change in the center wavelength of the broadband source input into an interferometer of the FOG produces drifts in a scalar factor associated with the acceleration and wavelength. Accurate and reliable measurements determined by detection of response signals from the optical sensors require a broadband light source outputting light with a center wavelength that does not drift around with time or other environmental changes. However, attempts in many environments to achieve such a stable broadband light source by stabilization and control (e.g., temperature stabilization or vibration dampening) of components proves difficult, expensive and oftentimes insufficient.

Therefore, there exists a need for optical sensing configurations and methods that improve wavelength stability of input broadband light used to interrogate an optical sensor which may include an FOG device.

SUMMARY OF THE INVENTION

In one embodiment, an optical system for producing a stabilized broadband light output to a sensor includes a broadband light source for producing broadband light signals and a splitter dividing the light signals into first and second portions along first and second output pathways, respectively. A controller module having a sweeping tunable filter coupled to the first output pathway of the splitter receives the first portion of the light signals prior to outputting respective filtered light portions to a comb filter and a wavelength reference element, wherein control circuitry is configured to evaluate detected signals from the comb filter and the reference element to generate a control signal output. A wavelength dependent variable attenuator coupled to the second output pathway of the splitter and the control circuitry receives the second portion of the light signals and the control signal output, wherein the attenuator is configured to modify the second portion of the light signals based on the control signal output, thereby providing the stabilized broadband light output.

For one embodiment, an optical system includes a broadband light source for producing broadband light signals and a splitter dividing the light signals into first and second portions. A sweeping tunable filter coupled to the splitter receives the first portion of the light signals prior to outputting respective filtered light portions to a comb filter and a wavelength reference element. Control circuitry configured to evaluate detected signals from the comb filter and the reference element generates a control signal output. A spectrum modifier coupled to the splitter and the control circuitry receives the second portion of the light signals and the control signal output, wherein the modifier is configured to adjust the second portion of the light signals based on the control signal output, thereby providing a stabilized broadband light output. A sensor element couples to the modifier and is configured to provide response signals from interrogation by the stabilized broadband light output that is unswept in time across wavelengths produced by the source. A receiver couples to the sensor element and is configured to detect and process the response signals.

According to one embodiment, a method of stabilizing broadband light output to a sensor includes generating a broadband light and dividing the light into first and second pathways, wherein a controller module wavelength scans light propagating in the first pathway prior to outputting respective filtered light portions to a comb filter and a wavelength reference element of the controller module. The method further includes generating a control signal output with control circuitry based on detected signals from the comb filter and the reference element. Modifying light propagating in the second pathway based on the control signal output produces the stabilized broadband light output.

DETAILED DESCRIPTION

Embodiments of the invention relate to sampling techniques which can constantly monitor a spectral output from a broadband source in order to control a central wavelength of interrogation light supplied by the source for input to a sensor. A first portion of light output from the broadband source passes through a controller module for spectral analysis and referencing to provide measurements that can be used as feedback to actively modify a second portion of the light from the source. This modified second portion thereby controls the central wavelength to ensure accurate determination of sensor response signals received at a receiver. In some embodiments, the sensor response signals may be from a fiber-optic gyroscope benefiting from the center wavelength being stabilized, as discussed herein.

FIG. 1shows a schematic process map of an optical system100for producing a stabilized broadband light output116. The system100includes a broadband light source102, a tunable filter104, first, second and third detector circuits106,108,110, a data processor112and a wavelength dependent variable attenuator114, which is controlled by signals generated with the data processor112to produce the output116derived from light signals provided by the source102. The source102, e.g., an amplified spontaneous emission (ASE) source, produces the light signals unswept in time across wavelengths and defining a broadband optical spectrum including wavelengths which may range, for example, at least 10 nanometer (nm) or at least 50 nm. The shape of the spectrum may change or drift over time. This instability causes changes in the center wavelength, which is critical in determining output from a sensor (see,FIG. 2). Therefore, control of the attenuator114by the processor112ensures that the center wavelength of light from the source102is maintained by wavelength and amplitude stabilizing the light prior to being output for use in interrogating the sensor.

In operation, a first portion of the light from the source102bypasses the tunable filter104and enters the attenuator114. A second portion of light from the source102passes through the tunable filter104prior to splitting into the first detector circuit106, the second detector circuit108, and the third detector circuit110that is optional. The tunable filter104may sweep across all wavelengths of the spectrum of the source102for whole spectrum measurement and analysis. Examples of suitable tunable filters include a piezoelectrically tunable Fabry-Perot (F-P) filter, a tunable acousto-optic filter, or a tunable interference filter. The processor112may control the tunable filter104to facilitate synchronization of measurements taken with the processor112based on signals from the detector circuits106,108,110.

The first detector circuit106includes a comb filter, such as an F-P etalon with fixed and known free spectral range, which produces a reference comb spectrum with peaks having a constant, known frequency separation equal to the free spectral range to provide an accurate frequency/wavelength scale. The second detector circuit108provides an accurate wavelength reference by, for example, passing light onto at least one fiber Bragg grating (FBG) with a known wavelength. Some embodiments can utilize a reference interference filter without a reference FBG by, for example, using a source envelope to identify one or more reference peaks in the comb spectrum itself for absolute wavelength referencing.

The signals detected in the first and second detector circuits106,108are simultaneously sampled, processed and compared in the data processor112, providing accurate and repeatable wavelength measurement across the whole measured spectrum. At the same time, the third detector circuit110measures spectral power of the light as received in the third detector circuit110for correlation to the wavelength measurement. This spectral power may therefore be derived from the first and second detector circuits106,108if power measurements are performed in addition to detecting the comb spectrum and Bragg wavelength. The detected signals in combination from the detector circuits106,108,110therefore enable monitoring and measuring the optical spectrum of the source102. If any changes in the spectrum are measured, signals generated by the data processor112can control the attenuator114to alter attenuation selectively for certain wavelengths of the first portion of light received at the attenuator114from the source102. The control of the attenuator114can maintain an identified center wavelength for the output116.

FIG. 2illustrates a block diagram of an exemplary optical sensor system200. The system200exemplifies an architecture employing concepts of the process map shown inFIG. 1. Components of the system200include a broadband light source202, a controller module219, a spectrum modifier214, a sensor and a sensor response detector and processor220. For some embodiments, the controller module219includes a tunable filter204, an F-P etalon211, a Bragg grating reference215, a comb detector206, a stable reference artifact detector208, and a data processor212.

Light from the source202travels to an initial tap or splitter203that splits the light into two paths. For some embodiments, the entire spectrum of the light from the source202passes continuously through the initial splitter203to the controller module219along controller optical fiber205and to a lead optical fiber207coupled to the modifier214. The tunable filter of the controller module219may provide the only wavelength scanning in the system200such that sensor interrogating light that does not pass through the controller module219may bypass any wavelength scanning of the light from the source202. The controller optical fiber205couples to the tunable filter204that wavelength scans the light to provide filtered light. A detection circuit splitter209couples the F-P etalon211and the Bragg grating reference215to the tunable filter204and divides the filtered light from the tunable filter204to each. A coupler or circulator213couples the Bragg grating reference215to the artifact detector208. The comb and artifact detectors206,208respectively sense outputs from the F-P etalon211and the Bragg grating reference215. The data processor212receives detected signals from the comb and artifact detectors206,208and evaluates a spectrum of the source202based on the detected signals as described heretofore.

The data processor generates control signals217input as operating instructions into the spectrum modifier214to regulate functioning of the modifier214. The control signals217may instruct the modifier214to adjust variable attenuation or amplification of certain wavelengths or dropping of certain wavelengths to ensure that the spectrum of the source202as received by the modifier214via lead optical fiber207is adjusted in a manner that produces a stabilized broadband light output through a sensing string216to the sensor218. For some embodiments, the stabilization may include wavelength and amplitude stabilization and may maintain an identified mean center wavelength. This stabilized broadband light output transmitted through the sensing string216interrogates the sensor218and may contain at one time substantially all wavelengths produced by the source202.

For example, the control signals217may instruct the modifier214to pass the light from the source202without alteration if the spectrum evaluated by the processor212already has the identified center wavelength. However, the control signals217may instruct the modifier214to attenuate wavelengths, such as 1530 nm to 1535 nm 10%, to obtain the identified center wavelength when the spectrum evaluated by the processor212has a shifted center wavelength different from the identified center wavelength. This example illustrates the ability to control broadband interrogation light with accuracy and in real time.

The sensor string216couples to the sensor218shown as an optical fiber sensing coil containing between 200 meters and 5.0 kilometers of fiber to form an interferometric fiber-optic gyroscope (IFOG). In operation, the stabilized broadband light output launches into the sensor218. Rotation of the sensor218affects the light, thereby generating response light signals. The response light signals from the sensor218propagate to the sensor response detector and processor220that then receives the response light signals for measuring rotation of the sensor218. Determinations of the rotation or other parameter obtained utilizing techniques as described herein may be transmitted as an output222to a user via, for example, a display or printout. Further, the output222may be used to generate a signal or control a device.

FIG. 3depicts a flow process300for stabilizing broadband light output to a sensor utilizing systems such as described herein. The process300begins at a light generating step302where light is emitted from a broadband source. At monitoring tap step304, dividing the light into first and second pathways occurs with light propagating in the first pathway being wavelength scanned to provide filtered light. The monitoring tap step304further includes outputting respective portions of the filtered light to a comb filter and a wavelength reference element. Instruction step306generates a control signal output based on detected signals from the comb filter and the reference element using control circuitry. The detected signals provide an indication of a spectrum of the light emitted by the source. Modifying light propagating in the second pathway occurs at spectrum stabilization step308based on the control signal output. Modified light produced at step308provides a stabilized broadband sensor interrogation light for interrogating a sensor.