Keysight N7752C and N776-C Family of Optical Attenuators

Optical Power Equalization with Ease

The Keysight N7752C and N776-C family of variable optical attenuators enables highly efficient optical transceiver and network integration testing. The new attenuator generation sports an on-board graphical user interface for easy control without software installation, accessible via GbE LAN and USB 2.0 connections. N7752C is a two-channel model for single-mode fiber equipped with two additional, independent optical power meters. N7764C is a four-channel model for single mode fiber, and based, like N7752C, on electrically controlled variable optical attenuator (VOA) modules. N7768C is a four-channel model for 50/125 µm multimode fiber, based on a miniature rotating filter design optimized to provide an insertion loss of typically only 1.6 dB including connectors. The attenuating filter is positioned in a collimated beam to maintain the same modal distribution across the entire attenuation range.

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 instrument 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 e.g. 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, like during probe adjustment.

Attenuation setting mode

In attenuation mode, the calibrated value of attenuation in dB can be 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.

The N7768C multimode attenuator can best use the factory calibration for the mechanical position of the attenuator filter. On the N7752C and N7764C, a user attenuation calibration is recommended at the wavelength and polarization state of the signal to be used and an automated procedure is provided to make this calibration.

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 N7768C rate can be set from 0.1 to 80 dB/s and to 1000 dB/s, the N7752C and N7764C 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.