Optical Attenuators

• Optical transceiver testing
• Optical network integration testing

Power Setting and Control Modes

In power setting mode, which is available on all N77-C variable optical attenuators, the power level at the attenuator output is set. The N7752C uses the feedback signal from a photodiode after a monitor tap to control the desired power level at the optical output. When power control is active, the instrument also automatically corrects for power changes at the input so that the output power level you set is maintained with ±0.02 dB typical repeatability.

The N7752C’s additional power meters can be used to measure the absolute power level from an external fiber to calibrate the offset from the attenuator’s output monitor, to correct for external insertion loss, like from connections, switches and couplers. Equipped with an analog voltage output, they can provide feedback for automated alignment applications, either linearly or logarithmically proportional to the optical power level. The new logarithmic mode is very helpful for tracking the signal level over a wide dynamic range, such as during probe adjustment.

Attenuation setting mode

In attenuation mode, the calibrated value of attenuation in dB is set directly, without use of the power monitor. This supports use with legacy software written for attenuators without power monitors or when the power monitor reading is impaired, as due to modulation or from a signal at another wavelength or in the reverse direction.

Advanced control of power change

Especially for testing devices that are sensitive to rapid power changes, the rate of change can be set on the instrument and applies to the attenuation mode. The N7752C can be set from 0.1 to 1000 dB/s. New with the N77-C attenuators is the ability to program a ramp of attenuation steps, including dwell time at each step, which can be either stepped automatically or synchronized with an external trigger. Trigger outputs are also available. This new functionality can be used instead of sending individual program commands for each step and aid in efficient synchronization with other instruments. In power control mode, during attenuation sweeps, as well as when activating the shutter mode, 1000 dB/s change rate is applied.

Calibration offsets for external losses

Comprehensive offset functionality allows you to calibrate the optical path in various test set-ups. There is an offset for the attenuation factor, and an independent offset for the output power level, to calibrate for losses due to patch cords, connectors, and switches. Additionally, wavelength and offset value pairs can be stored in a table to compensate for wavelength-dependent effects in the optical path of the set-up. This allows you to precisely set the optical power level directly at the input interface of your device under test.

With the extra optical power meter channel of the N7752C, calibration is even easier and more convenient. All power related offsets can be determined by a firmware function that reads a value from the reference power meter. The difference between the power value read by the reference power meter and the actual value of the attenuator is automatically stored as the offset.

New user attenuation-mode calibration

To calibrate the attenuation setting mode for a chosen source wavelength, a new self-calibration has been implemented. The calibration process, executed at the chosen wavelength and power higher than 0 dBm, and with the setup kept stable, will run for about 2 minutes. For using the attenuator at a different wavelength, the user calibration can be disabled, or the calibration can be executed at the new wavelength. The calibration data can be reenabled after a preset or reboot, or the configuration can be saved with enabled calibration.