Optoplex Corporation

Product Description - 2x8 Coherent Mixer with Single-ended PDs

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.

Features and Benefits of 2x8 Coherent Mixer with Single-ended PDs

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

Applications of 2x8 Coherent Mixer with Single-ended PDs

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

Optical Performance Specifications of 2x8 Coherent Mixer with Single-ended PDs

Parameter	Min	Typ.	Max	Unit	Conditions
Operating Wavelength Range	1525		1568	nm	C-Band
Tributary path delay	-		20	ps	Maximum skew between any path (XI, XQ, YI and YQ), Note 1
Tributary path delay variation			5	ps	Skew variation over operating conditions and aging
P/N path delay imbalance	-		2	ps	Between balanced pairs, over operating conditions and aging
Phase angle error	-5		+5	degree	Deviation from 90° angle between I and Q
Polarization extinction ratio	20			dB	To be measured over photocurrents
Optical return loss	27			dB	Each input
Responsivity of individual tributaries²	45			mA/W
Responsivity variation of individual tributaries over temperature			1.0	dB	Over operating case temperature range
P/N Responsivity imbalance³			0.85	dB	For Signal and LO path
Responsivity imbalance between tributaries ⁴			1.0	dB

Notes:

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).
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.
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:
Imbalance between XI, XQ, YI, YQ channels, for signal and LO path. The individual tributary’s P/N responsivity has to be averaged.