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How To Save Time When You Test an Integrated Coherent Receiver

Blog Post created by stmichel Employee on Feb 27, 2018

Setup of integrated coherent receiver test

 

 

The test of an integrated intradyne coherent receiver (#ICR) as defined by Optical Internetworking Forum requires many

parameters to test for each device. These devices can be tested in the above illustrated setup quickly and easily. Again the EVM concept offers a powerful tool to verify the overall quality of an ICR within a second. This setup simulates a #golden transmitter which has better performance than any production-series transmitter. Analyzing this signal in the same way as a normal transmitter signal can reveal impairments that reflect the intrinsic performance of the ICR under test (see left screens in the screen shot below) and therefore indicates limitations to the performance

that can be achieved.

 

In addition to the spectral display on the right screens, the #image suppression gives you an indication of distortions in the system and shows how well balanced your photodiodes are.

 

Screenshot - performance verification

 

For detection of complex modulated optical signals #OIF defined an electro-optical component typically described as integrated dual polarization #intradyne coherent receiver (ICR). This component contains optical and electro-optical components in one package.

 

The hybrid contains many components that need to be integrated and perform seamless as a black box coherent receiver.

The integrated component needs to be tested in research and in manufacturing.

 

The M8290A offers an application (M8290430A), to test this kind of devices and extract parameters that characterize the behavior of the component.

 

With the M8290A it is possible to test the component in an environment that is identical to the final application providing

highest confidence in the performance of the component:

 

  • This test is performed with the M8290A by generating a beat signal within the detection band to the ICR optical inputs using two continuous wave tunable laser sources.
  • This test is an excellent setup to verify the intrinsic performance of the ICR as it reflects noise impairments and all kinds of distortions.
  • The IQ diagram gives an indication on the noise and the distortion of the signal of the ICR created from a nearly perfect beat signal. The same parameters that are used to quantify the signal quality (EVM, IQ offset, IQ imbalance, Quadrature error) can be used to qualify the intrinsic performance of the component.
  • #Image suppression in a spectral display gives a good indication of presence of imbalances between channels and PIN diodes in the coherent receiver. A good image suppression and large common mode rejection ratio indicate a well balanced receiver.

 

ICR Image Suppresion Screenshot

 

Image suppression is an excellent indication of the presence of potential distortions within the optical receiver. An image

suppression in the order > 35 dB indicates high CMRR of well balanced PIN diodes and well de-skewed I-Q channels in the ICR under test.

 

EVM is an excellent indicator of the overall quality of a complex modulated signal. This concept is applied in that test by creating a beat signal in the ICR and analyzing it in the same way as a complex modulated signal. This emulates a kind of ideal stimulus of the ICR. With this test the EVM can be measured at a single frequency point along the receiver  bandwidth of the device under test and within the digitizer bandwidth. This measurement provides additional insight to the device under test, ensuring distortion-free measurements at each tested frequency point with good EVM.

 

ICR Frequency Response Screenshot

 

Integrated coherent receiver test provides most relevant test parameters as defined by OIF to characterize integrated coherent receiver components. The following test results are provided by the software:

 

  • Frequency response S21(f) for each tributary
  • Phase difference between I and Q as function of frequency for X and for Y polarization
  • Quadrature angles between I and Q for each polarization plane X and Y
  • Gain imbalances
  • Receiver skew values
  • Error vector magnitude (EVM % rms) over frequency (in addition to OIF)
  • Image suppression over frequency (in addition to OIF)

 

Coherent optical devices such as dual-polarization IQ modulators and intradyne coherent receivers need to be tested in their different development stages as well as qualified by the system integrators. The optionally available coherent optical device test software (M8290440A) provides a turn-key solution for the characterization of these devices. One user interface provides control of all instruments through a single software package. None of the tests requires

reconnecting the DUT, saving test time and reducing the uncertainty introduced by connecting and reconnecting the device. The coherent optical device test license provides:

 

  • S21 magnitude responses for XI, XQ, YI, and YQ
  • S21 phase responses for XI, XQ, YI, and YQ
  • IQ skew for X and for Y polarization
  • XY skew
  • Receiver IQ angle for X and for Y polarization (requires a two-laser setup)

 

Amplitude and Phase Response Screenshot

 

The test is based on generating a broadband multi-tone signal using a Keysight arbitrary waveform generator (AWG), e.g., M8196A. Comparing the received amplitudes and phases of each tone with the known original amplitudes and phases, the frequency and phase response of the device under test (DUT) can be reconstructed as shown in the figure below . From the measured phase response of each tributary, it is possible to calculate the individual group delays as well as the relative skews.

 

 Frequency and Phase Reconstruction Principle - Diagram

 

 

 

Setup for sequential testing of a device having both transmit and receive functionality, e.g., #IC-TROSA, #ACO, etc. The transmit-side is re-used after being characterized in the first step to generatethe test signal for the receive side.

 

    Block Diagram of Setup for Sequential Transceiver Testing

 

 

 

 

Setup for testing optical #IQ modulators. This test can also be used as an in-system calibration measurement. The resulting frequency responses can be exported as S-Parameter files and used for pre-distortion of complexly modulated data signals using QPSK or QAM formats.

 

Block Diagram of Setup for Testing Optical IQ Modulators

 

 

 

 

Setup for testing coherent receive devices as for example ICR modules. In contrast to the ICR test application (M8290430A), this test provides additionally the absolute phase response of each receiver path (XI, XQ, YQ and YQ) and allows to derive the respective group delay individually.

 

Block Diagram of Setup for Testing Coherent Receive Devices

 

 

 

 

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