Why does my OBR not require an annual calibration?

Luna Innovations' Optical Backscatter Reflectometer (OBR) is a high sensitivity, high spatial resolution optical instrument used to measure reflections as a function of distance in short-run optical networks. The measurement technique utilized by the OBR is swept-wavelength coherent interferometry. Using this method, the OBR is able to calculate accurate return loss and insertion loss measurements of optical networks from accurate measurements of optical power, distance traveled (derived from time delay of the signal), and optical frequency. 

To assure measurement accuracy, the OBR utilizes two onboard calibration devices: a molecular absorption cell used both as an optical frequency reference and a time delay reference, and a length of standard single mode fiber that serves as an optical power reference. These calibration devices, used with swept-wavelength coherent interferometry, provide continuous calibration of the instrument, meaning it is not necessary to send the OBR back to Luna Innovations or a third party calibration lab for regular calibration.

Wavelength Reference:

The OBR uses a wavelength sweep of a tunable laser to make its measurements, making it necessary to verify the laser output wavelength against a wavelength reference. Because in swept wavelength interferometry the time delay (which translates to distance traveled) is linked to optical frequency (the scaled inverse of wavelength) via a Fourier Transform, the wavelength reference also serves to calibrate time delay.

Gas molecular absorption cells have proven to be cost effective and extremely stable wavelength references and are widely used in fiber optic telecommunications test equipment. For the C and L band systems, Luna calibrates laser wavelength to a Hydrogen Cyanide (HCN) gas cell upon every measurement scan. NIST produces identical cells for use as optical spectral reference and tabulates the absorption wavelengths (NIST Standard Reference Material #2519). For O band systems, Luna uses a Hydrogen Fluoride (HF) gas cell as the wavelength reference. Although NIST does not produce or tabulate absorption lines for HF cells, HF absorption wavelengths are published in the high-resolution transmission molecular absorption (HITRAN) database (http://cfa-www.harvard.edu/hitran). HITRAN is a worldwide standards body that assembles and publishes spectroscopic data, and NIST bases their absorption line frequency values for their standard reference materials on the same database.

During each measurement scan, the OBR acquires transmission data through the gas cell and uses that data to refine the wavelength accuracy. This allows for continuous frequency calibration of the instrument. If the actual output wavelength of the laser given by the look-up table drifts beyond 300pm of its gas cell reference, a wavelength calibration error will occur in the software.

Optical Power Reference:

As the OBR measures return loss (RL) and insertion loss (IL), it is necessary to reference optical power. Luna Innovations' OBR uses the strength of Rayleigh scatter from a 0.5 m segment of Corning SMF-28E fiber inside the unit as an optical power reference. Because the fiber used in the OBR is manufactured in large volume to very tight criteria, this fiber section serves as a low-cost, stable and highly reproducible RL reference which the OBR uses to correct for long term drift in laser power, detector sensitivity and amplifier gain that might otherwise cause drift in the RL measurement. Furthermore, the OBR compares the measured instrument fiber Rayleigh scatter level to a set of glass-to-air interface reflection standards before each OBR is shipped to insure a high degree of absolute RL measurement accuracy and extremely good measurement reproducibility between instruments.

The optical power calibration to the Rayleigh scatter of the fiber inside the instrument is performed every time the user performs a calibration, making it unnecessary to send the unit back for calibration.

Calibration Services:

Luna understands that some customers require annual calibration, despite the information provided by Luna explaining the self-calibrating nature of the OBR.  Therefore we offer an annual calibration service as a convenience for the customer, not because it is required for proper operation.  Contact your sales representative or submit a ticket through the support portal to request a quote for calibration services.