GR1010881B - Vcsel based fiber bragg grating interrogator - Google Patents

Abstract

The present patent relates to a device for full spectral and temporal readout of Bragg gratings and microstructured fiber optic sensors. The device consists of one or more Vertical Cavity Surface Emitting Lasers (VCSELs) and the accompanying electronic drive subsystems that accurately and stably control the optical emission of the laser that constitutes the optical input to one or more microstructured sensors or Bragg gratings. The device further consists of a free space or fiber optic optical circulator. The reflected light from the sensors enters the optical circulator and is redirected to a photodiode that detects the intensity of the reflected light and is electronically synchronized with the VCSEL so that both spectral information and high-speed temporal information are recorded when the VCSEL is operating in a spectrally locked position. The accompanying electronics of the device are responsible for the precise and repeatable photodiode synchronization with the LED.

Description

[0001] Description

[0003] Patent: Bragg Grating (FBG) reader based on Vertical Cavity Surface Emitting Laser (VCSEL)

[0005] Patent area

[0007] The present patent relates to the field of fiber optic sensor readers based on Fiber Bragg Gratings (FBGs).

[0009] Explanatory report

[0011] Fiber Bragg Grating (FBG) based fiber optic sensors are considered to be the most commercially successful of their kind in the last 20 years. These sensors have found a wide range of applications as high-performance and reliable sensors in structural integrity monitoring, medicine, engineering and a variety of other applications. Such fiber optic sensors are now particularly widespread in critical high-cost applications that require high performance, such as structural integrity monitoring of bridges, nuclear reactors, spacecraft gyroscopes, water dam pressure monitoring and medicine among many other applications. The common denominator in the applications in which these sensors are used is the high performance requirements of the sensor and their high cost. The biggest obstacle to a wider use of Bragg gratings is the prohibitive cost of their reading device and specifically the lack of inexpensive spectral reading devices for these sensors.

[0013] The cheapest FBG sensor reader on the market operates on the basis of the edge detection method. In this method, an optical source of narrow (laser) or broad (LED) spectral content is incident on the spectral edge of a Bragg grating and the reflected light is monitored by a photodiode. However, there are some serious disadvantages to this method.

[0014] First, the method is unable to obtain spectral information from the sensor and can only monitor one sensor per reading sequence. Furthermore, it is particularly susceptible to variations in light intensity and drift, which can lead to erroneous measurements. Finally, the operation of this method assumes that the sensor spectrum remains unchanged during use, which may not be the case in many applications.

[0016] Another method of reading Bragg grating sensors is based on Charge Coupled Device (CCD) devices and is essentially a spectrometric method. A high-resolution grating is used to analyze the incident light which is imaged onto a linear CCD sensor. The light source is usually of a broad spectral range within the spectral range of the optical sensor. This method is limited by the silicon lithography, from which CCD devices are usually made and cannot detect optical signals at wavelengths above 1000 nm. This implies the inability to use inexpensive peripherals that are abundant around the 1550 nm region due to optical telecommunications. The use of devices using semiconductors other than silicon, such as e.g. InGaAs, significantly increases the cost of the method, making it unattractive.

[0017] Finally, the most common method of reading FBG sensors, which is also the most expensive, is the use of a variable wavelength laser. In this method, a variable wavelength and narrow spectral width laser is scanned throughout its spectral range, typically around 100 nm, near the telecommunications window of the C region. The reflected spectrum from the sensor(s) is directed to a photodiode that reconstructs the spectrum. Such a device can support the parallel reading of multiple sensors that are spectrally separated from each other, while providing excellent signal-to-noise ratio in measurements and very good spectral resolution. However, these devices are characterized by their high cost, which is a brake on the wider dissemination of Bragg grating technology to a wider audience or in applications with a higher sensitivity to the performance/cost ratio.

[0019] The present invention described herein aims to provide a low-cost, ultra-high resolution, high dynamic range FBG sensor reader, with the ability to fully scan the sensors, as well as the ability to use the edge detection technique with high response speed in one device. The device can operate spectrally on or around the optical telecommunications window C or in the wider and extended region thereof. The device is based on the use of one or more vertical cavity surface emitting lasers (VCSLs), detection photodiodes and circulators and consists of the necessary electronic subsystems for its accurate, stable and repeatable operation for extended periods of time.

[0021] Description of the invention

[0023] The present invention relates to a fiber optic sensor reader based on Fiber Bragg Gratings (FBG) or microstructured sensors (100, 101). The FI device is based on and consists of one or more Vertical Cavity Surface Emitting Lasers (VCSL) of variable wavelength and optical power (120), one or more photodiodes (PDs) (123), a motherboard housing the required power electronics (121), a microcomputer or microcontroller (122), one or more optical circulators (110) and the necessary optoelectronic connections (102, 124). The device may be powered by a battery (140).

[0024] The device of the present invention uses as a light source one or more Surface Emitting Lasers (SELs) of variable wavelength and optical power (120), emitting around or on the telecommunications window C. The optical output power is varied by changes in the current flow and the wavelength is varied by changes in the voltage at the ends of the SELs. The stability of the selected wavelength and optical power is ensured by the integration of temperature control in the device (201) and feedback from the SELs via an integrated thermistor. The photodiode of the device is of variable gain, by converting the current flowing through its ends into voltage and amplifying it (205), providing variable levels of optical sensitivity and dynamic range.

[0025] The control of the operating parameters of the LEC(s) as well as the photodiodes(s) is implemented through the integration of discrete electronic subsystems on the device's motherboard (121). The motherboard (mainboard) (121) incorporates the LEC(s) temperature control and monitoring subsystem (201) which is based on a commercially available temperature control integrated circuit (TEC). The TEC subsystem is responsible for operating the LEC(s) at a constant temperature, providing stable and repeatable optical power and spectral emission position adjustment. In addition, the motherboard incorporates the isolated current source (isolated current source) electronic subsystem (202), the purpose of which is to provide a constant flow of electric current to the LEC(s), in order to achieve stable and precise adjustment of the laser's optical output power. The isolated power source is implemented using an operational amplifier (OpAmp) and digital to analog converters (DACs). The wavelength change of the LEC(s) is implemented through the electronic voltage control subsystem (203) which is based on the Direct Digital Synthesis (DDS) technique. The motherboard additionally integrates the electronic photodiode driving subsystem (205), which is based on the Programmable Gain Amplifier (PGA) architecture. The motherboard also includes the necessary power supply electronic subsystems (200) consisting of converters and voltage regulators. Finally, the protection circuit of the LEC(s) (204) is also integrated into the motherboard.

[0027] The motherboard is connected to a microcomputer or microcontroller, hereinafter referred to as the processing unit (122), whose role is to control the electronic subsystems of the motherboard and to interface the device with the end user or with a terminal (130). The drivers of the electronic subsystems on the motherboard are programmed in the processing unit and a linear or graphical user interface is programmed for the interface with the user (130). In the case of a microcomputer implementation, a graphical user interface is used for the interface with the end user, while when a microcontroller is used, the data of the device is transmitted wirelessly to the display and control terminal device which may be a personal computer or a mobile phone, among others.

[0029] The device may include one or more optical input ports for mounting fiber optic sensors (110, port 2). The optical output of the LFCC is directed to the input of the optical circulator, and the output of the circulator constitutes the optical input of the device. The return of the circulator (110, port 3) is directed to the amplified photodiode, providing detection of the light reflected from the input of the device.

[0031] The device operates in two distinct modes. The spectral scanning mode and the edge-detection mode. In the spectral scanning mode, the LEEKK is driven by the variable voltage subsystem to perform a full or partial spectral scan and the reflected spectrum is detected by the amplified photodiode which is electronically synchronized with the LEEKK scan to provide spectral information. In the edge-detection mode, the LEEKK is positioned at a predetermined position in the spectrum and the reflected optical signal is continuously detected by the photodiode. The edge-detection mode can be automatically activated after a full spectral scan to: 1) identify the spectral location of Bragg gratings and then, 2) define the spectral position of the LEEKK in the FBG and finally, 3) activate the detection of optical power change by the photodiode. In this way, the device can be used for detecting an acoustic signal.

[0033] The device of the present invention combines the full spectral detection capability of the variable wavelength laser method, with extremely high resolution and dynamic range, together with the speed of the edge detection method, in a low cost and high performance device, due to the low cost of the semiconductor LECKK light source.

Claims (8)

1. Claims This patent claims: 1. A device for reading fiber optic sensors of the Fiber Bragg Grating type or microstructured fiber optic sensors, consisting of: a Vertical Cavity Surface Emitting Laser (VCSEL) of variable wavelength and variable optical power, used as a source for sensor readout and operating on or near the telecommunications window C or its extended version; a photodiode used to receive the reflected optical signal from the sensor(s); an optical fiber circulator used to distribute light signal from the LES to the sensors and from the sensors to the photodiode; power management and distribution electronics? the temperature control and regulation electronics of the LEEKK? the electronics of the regulated, isolated power source? voltage regulation electronics which are but not limited to Direct Digital Synthesis architecture; the protection circuit of the EEKK? the photodiode driving electronics which are but not limited to the Programmable Gain Amplifier architecture; digital control and interface electronics that may use but are not limited to I2C, SPI, or USB interfaces; a microcomputer that controls all the electronics, processes the photodiode data, and displays the sensor output via a graphical user interface. 2. The device of claim 1, wherein more than one LED, more than one photodiode, and more than one circulator are incorporated into the device, providing multi-channel operation. The device of claim 2, wherein a microcontroller controls the electronic subsystems and provides a data interface to an external device. 4. The apparatus of claim 1, wherein the LEC operates in the extended band of optical fiber telecommunications. The device of claim 1, wherein the device operates via battery power. 6. The device of claims 1 or 2, wherein the reading of the fiber optic sensors takes place through full or partial scanning of the optical spectrum of the LFCC. 7. The device of claims 1 or 2, wherein the reading of the fiber optic sensors takes place via the edge detection technique. 8. The device of claims 1 or 2, wherein the edge-reading technique is used after scanning an FBG sensor to perform acoustic wave detection.