This page defines the requirement of a special coherent receiver with 8 single-ended photodiode outputs, per customer request.

The coherent mixer, consisting of two polarization-diversified 90deg optical hybrids, a polarizing beam splitter and a beam splitter, is exactly same as existing DP-QPSK coherent mixer, and therefore offering same optical performance.

Figure 1, Optoplex’s existing 2x8 DP-QPSK Coherent Mixer

To achieve the required configuration, functionalities and performance, 8 individual single-ended photodiodes are used to replace the 8 output collimators in the existing coherent mixer design.

Figure 2, Optoplex 2x8 coherent receiver with 8 single-ended PD outputs

Figure 3， Functional block diagram of the coherent receiver with single-ended PD output

The 8 single-ended photodiodes are completely independent of each other – each one has its own and separate ground to provide the user the flexibility for independent monitoring and control.

• Free-space optics based 90deg optical hybrid
• Accurate 90deg phase difference, small temperature, wavelength and polarization dependence
• Superior optical performance (IL, TDL, PDL, Skew, etc.)
• Low dark current
• High CMRR
• High PER

• Coherent Doppler LIDAR system
• Coherent detection in fiber sensing
• Coherent detection in OCT and other biomedical sensing/imaging systems
• Coherent spectroscopy instrumentation
• Coherent detection in optical communications

Notes:

1. Overall delay between signal input or LO input and the respective electrical output. TIA gain set to medium gain. Use a heterodyne measurement setup. Sweep the beat frequency from 0.1 – 10 GHz and derive the phase response between any path (XI, XQ, YI, and YQ).
2. This is the worst case responsivity of one individual tributary from LO or Signal input, to one of four outputs, thus including inherent and excess loss. The responsivity is to be averaged over the P and N components of each tributary. To measure the average responsivity of the Signal path, a polarization scrambled signal shall be used.
3. Imbalance between P/N pair of one tributary for Signal and LO path. The imbalance is related to CMRR_DC. The definition of CMRR_DC is:
   
4. Imbalance between XI, XQ, YI, YQ channels, for signal and LO path. The individual tributary’s P/N responsivity has to be averaged.