• Input/Collimation: it leads the incoming optical signal to a Thorlabs F230SMA-A aspheric collimator.² This aspheric lens ensures that the optical signal impinging on the following block consists only of parallel beams regardless of the light launching conditions at the optical input.

• Wavelength separator: the incoming optical signal from the aspheric collimator impinges on a Newport 10D20ER.1 concave mirror³ and then it is directed to a reflective holographic grating. This holographic grating, a NT43-225 model from Edmund Optics⁴ with a resolution of 2400 grooves per millimeter and suitable for the visible region, is the core component of the spectrometer: it separates the incident light into its constituent wavelength components, being each wavelength reflected into a different direction according to the grating equation⁵

• Detector: the detector consists in a Thorlabs PDA-55 Si photodetector.⁸ It converts the optical beam coming from the wavelength separator into an electrical signal. Finally, the converted electrical signal is sent to the control and processing unit.

• Data acquisition unit: it is responsible for digitalizing the electrical output of the spectrometer (previously conditioned by the signal conditioning circuit) and for processing this digitalized signal. Depending on the strength of the electrical output, the data acquisition unit sends the appropriate control signals (via I/O signal generation unit) to the signal conditioning circuit to increase or reduce the gain accordingly.

• Communication unit: it links the DSP and the “OSAgui” Windows application installed on the PC together (the physical connection is made through the parallel port). It interprets the commands sent by the “OSAgui” Windows application (according to the requests made by the user) and sends the measurement results provided by the data acquisition unit to the “OSAgui” Windows application (so as to display them to the user).

• Position control unit: it is responsible for controlling the angular position of the stepper motor during the spectral sweep by sending the appropriate PWM signals.

• Calibration unit: it calibrates the spectrometer when the prototype is started up. For this purpose, the DSP changes sequentially the angular position of the stepper motor until the optocoupler triggers a signal; this angular position is assigned to the reference wavelength.

• Control unit: it controls and coordinates each one of the units, monitors the status of the DSP, and synchronizes in real time the different tasks that have to be carried out.

• Communication layer: it performs the required operations to establish, maintain and stop the bidirectional communication with the DSP. It sends the pertinent commands to the DSP and receives the data containing the measurement results from the DSP.

• Arithmetic layer: it processes the data provided by the communication layer. Its main task is to compensate for the variation in the obtained spectral power with the wavelength (see subsection 2.1 and Fig. 3 for further details). If requested by the user, it also filters the compensated measurement results and performs a Fast Fourier Transform.¹⁰

• Data storage layer: it stores the data provided by the arithmetic layer in the hard disk and vice versa.

• Interface layer: it is the layer that interacts with the user. On the one hand, it prompts the user to enter the analysis parameters and other required data in the fields provided by the corresponding dialog box. On the other hand, it displays the obtained measurement results to the user.

• Control layer: it distributes the tasks to the corresponding layers and coordinates them, processes the data for its graphical representation by the interface layer, and monitors and handles every error and exception thrown during a certain operation.

• Laser diode: it consists in a Mitsubishi FU-427SLD-F1M54 module that operates at 1310 nm.¹² The laser diode transforms the electrical pulses generated by the control and processing unit into optical pulses.

• Optical circulator: this is a polarization insensitive optical circulator from AC Photonics, which transfers the optical pulses from its first port to its second port (i.e., to the output of the OTDR) and the backscattered power from its second port to its third port (i.e., to the photodetector).¹³

• Photodetector: this is a very high speed Thorlabs D400FC InGaAs photodetector.¹⁴ It converts the backscattered power from the optical fiber under test into an electrical signal, which is sent to the electronic unit.

• Signal conditioning circuit: this dedicated electronic circuit amplifies and makes suitable the electrical output from the photodetector before it is fed into the A/D converter. For such a purpose, it features a THS3001 high speed amplifier from Texas Instruments.¹⁵

• Analog-to-digital (A/D) converter: the main component is an ADS5410 integrated circuit, which is mounted on an ADS5410EVM evaluation module that provides all the necessary interconnection elements and sockets.^{16, 17} The A/D converter samples the adapted electrical signal from the signal conditioning circuit and sends it to the control and processing unit via 40-pin flat ribbon cable. The sampling rate is obtained from the clock sent by the control and processing unit through a coaxial RF cable.

• Data acquisition unit: it reads the digitalized data from the A/D converter and processes them according to the parameters given by the configuration unit (this digitalized data represents the backscattered power as a function of the distance, from now on, the OTDR trace). Due to high speed requirements, the data acquisition unit makes use of the Enhanced Direct-Memory-Access built-in controller (via Memory Expansion Connector).

• Communication unit: it links the DSP and the “Otdr” Windows application installed on the PC together. Its purpose is to receive the commands sent by the “Otdr” Windows application and to send the OTDR traces back to the Windows application. The physical connection is made through the USB port.

• Laser diode pulse generation unit: it sends the signal pulses to the optical unit by means of the General-Purpose Input/Output peripheral (via Peripheral Expansion Connector). The duration of the pulses is set according to the parameters given by the configuration and control unit.

• A/D converter and laser diode biassing unit: it controls through the Peripheral Expansion Connector the biassing of the A/D converter and the laser diode.

• A/D converter control unit: it generates the clock signal and sends it to the A/D converter through the Peripheral Expansion Connector.

• Configuration and control unit: it initializes the DSP, interprets the commands provided by the communication unit, and configures and coordinates the memory, the external peripherals, and the rest of the units with the appropriate parameters in order to carry out the reflectometry measurements.

• Communication layer: it is responsible for the bidirectional communication between the “Otdr” Windows application and the DSP. On the one hand, it sends the pertinent commands to the DSP to carry out the measurements; on the other hand, it receives the OTDR trace from the DSP.

• DSP start-up layer: this layer dumps the DSP routines to the RAM of the DSP each time the prototype is switched on. This is accomplished by running a script from the Code Composer Studio.

• Arithmetic layer: first of all, it converts the trace provided by the communication layer to logarithmic units (for efficiency reasons the DSP only handles linear data). Afterwards, it filters the trace in order to increase the signal-to-noise ratio and, finally, scans the trace for events. ¹⁹

• Data storage layer: on the one hand, it stores/retrieves the trace in/from the hard disk; on the other hand, it performs the corresponding operations in order to export the measurement results to a spreadsheet, such as Microsoft Excel, or to MATLAB.

• Interface layer: it allows the user to enter the analysis parameters in a very intuitive way (using very simple dialog boxes, menus and toolboxes). It also displays the OTDR trace and information about the detected events (fiber losses, insertion losses, etc.) and it provides the user with tools such as markers that enable the analysis of single events.