Vcsel based fiber bragg grating interrogator

Abstract

In the present disclosure an apparatus for the full spectral and temporal interrogation (measurement) of Fiber Bragg Grating and microstructured optical fiber sensors is described. The apparatus is comprised of one or more wavelength tunable Vertical Cavity Surface Emitting Lasers (VCSELs) and the accompanying driving electronics which control in an accurate and stable manner the emitted laser light which is used as input towards a single or multiple Fiber Bragg Gratings or microstructured optical fiber sensors. The apparatus is further comprised of an optical circulation system, either in the form of an optical fiber circulator or in the form of a free space circulator. The reflected light from the FBG enters the optical circulator and is routed towards a photodiode which reads the intensity of the optical signal and is synchronized with the VCSEL to acquire data both during a VCSEL wavelength sweep, providing spectral data, or individually with the VCSEL wavelength locked, to provide very high-speed sensor readout. The apparatus accompanying electronics are responsible for the accurate and repeatable VCSEL and photodiode synchronization.

Description

[0001] DESCRIPTION

[0002] VCSEL BASED FIBER BRAGG GRATING INTERROGATOR

[0003] Field of the disclosure

[0004] This disclosure relates to the field of optical fiber sensor interrogator devices and in particular to the field of Fiber Bragg Grating interrogator devices.

[0005] Background

[0006] The Fiber Bragg Grating optical fiber sensors are considered to be the most commercially successful optical fiber sensor in the last 20 years. Such sensors have found an increasing number of applications as high performance and reliable components in structural health monitoring systems or general sensing devices. Optical fiber sensors are now widely employed in generally mission critical high performance, high-cost applications, such as bridge health monitoring, nuclear reactor monitoring, spacecraft gyroscopes, dam pressure monitoring, in medicine, among many other applications. The common denominator, in the applications where optical fiber sensors are used, is the requirement for high performance and the high cost. The biggest obstacle towards a wider adoption of the Fiber Bragg Grating sensor is the prohibitive cost of the sensor readout unit, or the lack of low-cost systems with spectral scanning capabilities.

[0007] The current lowest cost FBG interrogation solution is the so-called edge filter detection method. In this method, the narrowband light of a laser source is shone towards the spectral edge of a FBG and the reflected light is monitored by means of a photodiode. The laser can also be substituted by a Light Emitting Diode (LED), driving the costs further down. In this method however, several major drawbacks are present. First, the method completely lacks the capability of a full spectral sweep and can only interrogate a single sensor. Moreover, it is highly affected by light intensity fluctuations and light source drifting, which can provide erroneous sensor reading. Finally, it assumes that the spectrum of the interrogated FBG does not change under loading, which is not true for many applications and can lead to further misleading measurements.

[0008] Another method of FBG interrogation is the Charge Coupled Device measuring method, which is practically a spectroscopic method. A high-resolution grating is used to analyze the input optical fiber light into its spectral components which are imaged across a linear CCD array. The light source is usually a broadband light source at the desired wavelength range. This method is limited by the CCD silicon lithography, which does not detect signals above lOOOnm. This means that the low cost 1550nm components around the telecommunications window are not accessible and when using other semiconductor linear arrays, such as InGaAs, the cost of such interrogators is significantly increasing.

[0009] Finally, the most widespread FBG interrogation solution, which is also the most expensive one is based on tunable lasers. In this method a tunable narrowband laser is scanned along a large wavelength range, typically around lOOnm around or near the optical telecommunications C- band. The reflected spectra from the sensors are directed towards a photodiode that reconstructs the spectrum. This type of apparatus offers multiple optical sensor interrogation and very good signal to noise measurements as well as very good spectral resolution. However, such interrogators are characterized by their very high cost that has prevented wider adoption of the technology in applications that are cost-sensitive.

[0010] The invention described herein is aiming at providing a low-cost apparatus for FBG interrogation capable of very high resolution and dynamic range full spectrum measurements and also provide the capability of edge filter detection at very high speeds, in one single apparatus, operating around or near the optical telecommunications C-band. The apparatus is based on the use of a Vertical Cavity Surface Emitting Laser as the laser light source, light detection photodiodes and circulators along with the necessary electronics that are designed to provide stable and accurate operation over long operation times.

[0011] Patent description

[0012] The current disclosure describes an apparatus used for FBG or other microstructured optical fiber sensor (100,101) interrogation. The apparatus is based on and comprised of one or more wavelength and output power tunable Vertical Cavity Surface Emitting Lasers (VCSELs) (120), one or more photodiodes (123), a printed circuit board, hereafter referred to as the mainboard (121), housing the required driving and power electronics, a microcontroller or computer module (122), one or more optical circulators (110) and the accompanying optoelectronic connections (102, 124). The apparatus might be powered by an external or internal battery (140).

[0013] The apparatus in the present disclosure is using as a light source one or more wavelength tunable VCSELs (120) emitting on or near the optical telecommunications C-band window. The VCSEL(s) are tunable both in terms of optical power and in terms of optical emission wavelength. Optical power is adjusted by means of current flow through the VCSEL and emission wavelength is adjusted by voltage changes at the VCSEL's terminals. The stable power emission and wavelength position of the VCSEL(s) during operation is ensured by the embodiment of temperature control in the apparatus (201) and feedback through the VCSEL's built-in thermistors. The photodiode in the apparatus is of adjustable gain by means of electrical current amplification and voltage conversion (205) across its pins, providing adjustable levels of optical sensitivity and dynamic range.

[0014] The control of the VCSEL as well as the photodiode operational parameters is embodied by the utilization of discrete driving electronic subsystems present in the apparatus mainboard (121). The mainboard (121) includes the temperature monitoring and control subsystem

[0015] (201), based on Temperature Control (Thermoelectric Cooler -TEC) integrated circuit. The TEC subsystem is responsible for the operation of the VCSEL at a stable temperature which in turn provides the required optical stability of laser operation. Additionally, the mainboard includes the isolated current source electronics subsystem (202) which is used to provide stable current towards the VCSEL for precise and stable optical power output. The isolated current source

[0016] (202) is based on Operational Amplifier (OpAmp) and Digital to Analog Converters (DACs). The VCSEL wavelength tunability is realized by the voltage control subsystem (203) which is based on Direct Digital Synthesis (DDS). The mainboard also includes the photodiode driving electronics (205), which are based on a Programmable Gain Amplifier (PGA) design. The mainboard includes the required power supply subsystems (200), comprised of a buck converter and voltage regulators. Finally, the mainboard houses the VCSEL protection circuit (204).

[0017] The mainboard is connected either to a microcontroller, or a microcomputer, hereafter referred to as the 'processing unit' (122), whose role is to control the mainboard electronics and interface the apparatus operation towards the end user or a terminal operation station (130). The electronics subsystems drivers are coded in the processing unit and a command line interface or graphical user interface is developed for human operation of the apparatus (130). In the case of the embodiment of a microcomputer, a graphical user interface is used for human interaction with the apparatus and when a microcontroller is used, the data between the apparatus is transmitted wirelessly to the end-user device such as personal computer, mobile phone, tablet, where the graphical user interface and processing takes place.

[0018] The apparatus may contain one or more optical inputs for optical fiber placement (110, port 2). The output of the VCSEL light source is directed towards the input of the circulator circuit and the output port of the circulator is the optical input of the device. The return port of the circulator (110, port 3) is directed towards the amplified photodiode, providing reflected signal detection from the device input port.

[0019] The apparatus works in two different modes. Spectral acquisition mode, or edge-detection mode. In the first mode, the VCSEL is spectrally driven by the voltage control subsystem to perform a full or partial spectral sweep and the spectrum is acquired by the photodiode which is synchronized electronically to the sweeping signal. In the edge-detection mode the VCSEL is set at a predefined spectral position and the optical signal is continuously acquired by the photodiode. The edge detection mode may be automatically activated after a full spectral sweep in order to: 1) acquire FBG peak position and then, 2) set VCSEL wavelength and 3) trigger photodiode detection. In this mode it can be used for acoustic emission detection after a predefined trigger event.

[0020] The apparatus in this disclosure combines the spectral acquisition capabilities of the tunable laser method, along with a very high resolution and dynamic range and the speed of the edge detection method, at a low-cost device, due to the relatively lowcost of the VCSEL light source.

[0021] Figure description

[0022] The above features, including additional ones, may be better understood by the following non-limiting description of the illustrations appended.

[0023] Figure 1 illustrates as an example a schematic embodiment of the optical fiber sensor interrogation apparatus comprised of the Vertical Cavity Surface Emitting Laser (VCSEL) light source, connected to a circulator on its input port (1), the optical fiber sensor(s) connected the circulator output port (2) and a photodiode (PD) receiving reflected light from the third (3) circulator port. The VCSEL and the photodiode are controlled by the apparatus mainboard which contains the driving, power and synchronization electronics and everything is controlled by a processing unit. The processing unit is also responsible for interfacing the apparatus functionality and readings to the end user by a user interface generated either internally or externally to the apparatus.

[0024] The reference symbols in this Figure designate the following elements: 100 - Fiber Bragg Grating (FBG) optical fiber sensor.

[0025] 101 - Second or multiple optical fiber sensor, either FBG, or in the case of terminal sensor, possibility of microstructured optical fiber sensor.

[0026] 102 - Optical fiber.

[0027] 110 - Optical fiber or free space optical circulator. Optical input port designated as (1), optical output port as (2) and return port as (3).

[0028] 120 - One or more Wavelength tunable and power adjustable Vertical Cavity Surface Emitting Laser(s) (VCSEL), emitting on or around the optical telecommunications C-band or in the extended telecommunications windows.

[0029] 121 - Mainboard of the apparatus containing the required driving, synchronization and power electronics for the operation of the apparatus.

[0030] 122 - Apparatus processing unit. Either a microcomputer or a microcontroller for controlling the totality of the signals to and from the mainboard and in the case of microcomputer, processing the data locally and visualizing the results. In the case of microcontroller, the data is processed and visualized in a remote device. In both cases, there is wireless connection capability in the form of WiFi, Bluetooth, or Long-Range (LoRa) Wireless.

[0031] 123 - One or more Photodiode(s) capable of detecting optical signals near or around the optical telecommunications C-band or the extended telecommunications window around the C-band.

[0032] 124 - Electronic signal and electrical connections.

[0033] 130 - User Interface, either generated locally on the apparatus, or externally on a user device.

[0034] 140 - Optional battery

[0035] Figure 2 illustrates as an example a schematic embodiment of the mainboard layout comprised of the electronic subsystems required to operate the apparatus in a stable, reliable and repeatable fashion. The mainboard embodies the power conversion, regulation and distribution electronics to provide each subsystem the required voltage and current. The Vertical Cavity Surface Emitting Laser (VCSEL) requires three different independent subsystems to operate. The temperature control subsystem (TEC), the isolated current control subsystem that controls output optical power of the VCSEL and the Direct Digital Synthesis Voltage Control subsystem that tunes the VCSEL wavelength. The signals of all these subsystems converge towards the VCSEL protection circuit which is the physical connection point of the VCSEL to the mainboard. The photodiode is driven by a Programmable Gain Amplifier electronics subsystem which provides adjustable gain to varying light intensity input. The mainboard is controlled through the interface to the processing unit.

[0036] The reference symbols in this Figure designate the following elements:

[0037] 121 - Mainboard of the apparatus containing the required driving, synchronization and power electronics for the operation of the apparatus.

[0038] 200 - Power management and distribution electronics.

[0039] 201 - VCSEL temperature control electronics subsystem.

[0040] 202 - Isolated current source electronics subsystem.

[0041] 203 - Direct Digital Synthesis voltage control electronics subsystem.

[0042] 204 - VCSEL protection circuitry.

[0043] 205 - Programmable Gain Amplifier photodiode driving electronics subsystem.

[0044] 206 - Digital control electronics.

[0045] 210 - Interface to processing unit.

[0046] 211 - Physical connection to photodiode.

[0047] 212 - Physical connection to VCSEL.

Claims

CLAIMS1. An apparatus used for Fiber Bragg Gratings or microstructured optical fiber sensors interrogation comprised of: a wavelength tunable and power adjustable Vertical Cavity Surface Emitting Laser (VCSEL) used as the laser light source for the sensor input, operating near or around the optical telecommunications C-band; a photodiode used for receiving the reflected optical signal from the sensor(s); an optical fiber circulator used to deliver light from the VCSEL to the sensor and from the sensor to the photodiode; the power management and distribution electronics; the VCSEL temperature control electronics; the isolated current source electronics; the voltage control electronics that are of but not limited to the Direct Digital Synthesis architecture; the VCSEL protection circuitry; the photodiode driving electronics that are of but not limited to the Programmable Gain Amplifier architecture; the digital control electronics that may be but not limited to the I2C, SPI, or USB architecture; a microcomputer that controls all the electronics, processes the photodiode data and visualizes sensor output through a graphical user interface;2. The apparatus of claim 1, wherein more than one VCSELs, more than one circulator and more than one photodiode may be embodied in the apparatus, providing multi-channel operation.

3. The apparatus of claim 2, wherein a microcontroller controls the electronics subsystems and interfaces data to an external device.

4. The apparatus of claim 1, wherein the VCSEL operates at any of the extended optical telecommunications bands around band C.

5. The apparatus of claim 1, wherein the apparatus is battery operated.

6. The apparatus of claim 1 or claim2, wherein the optical fiber sensor interrogation takes place by full or partial optical spectrum sweep.

7. The apparatus of claim 1 or claim2, wherein the optical fiber sensor interrogation takes place by the edge detection technique.

8. The apparatus of claim 1 or claim 2, wherein the edge detection technique is used along an optical fiber FBG sensor to perform acoustic emission detection measurements.

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