（90° optical hybrids）

1、 Description

The COH24 is a single polarization 90° optical hybrid that enables to extract phase and amplitude fromasingle polarization signal by performing four 90° phase stepped interferences between the signal and aLocalOscillator (LO). The COH24-X is a new generation optical hybrid with improved performances (among otherthings, a wider operating wavelength range and muchbetter dual uniformities).

The COH28 and COH28-X are based on two single polarization 90° optical hybrids that enable to extract phase, amplitude and polarization from a signal with any polarization.

These products are purely passive and athermal.The COH24-X and COH28-X also feature a phase tunability option in order to perfectly adjust the 90° ofthehybrid depending on the wavelength used.

2 – Block diagram

Below is a block diagram showing how the four 90° phase stepped. Interferences are done in the opticalhybrid.

The graphs above compare the dual uniformities, a key parameter of 90° optical hybrids, for the SignalandLO of a standard hybrid (COH) and a new generationone (COH-X). We can clearly see that COH-X exhibits much better dual uniformities over a wider range.

7 – Phase tuning option

The 90° phase shift of the hybrids depends slightlyon the wavelength. For example, it increases of about 2°to 3° over the C-band. That means that if at 191THzthe phase shift is 88.5° it will be approximately91.5°at197THz (see next paragraph for some typical measurements).

For some applications it can be useful to have a very precise 90°phase shift whatever the wavelengthyouare working at. That is what the phase tunability option on COH-X enables. With this option, two voltages(V1 and V2) between 0V and 4V can be applied on thehybrid. One will increase the phase shift when thesecond one will decrease it. Hence for any wavelength in the OWR the phase shift can be adjusted to beprecisely equal to 90°.

Below are the packaging of a COH24-X with the phasetuning option and a typical measurement of the phaseshift versus voltage for a COH24-X device:

Graph: phase vs voltage for a COH24-X with phase tuning option

8 – Typical measurements

The two following diagrams show the typical behavior of the 90° phase shift. On the first one, one canseethat the variation of the phase shift versus the temperature is very small (less than 1°). Hence the hybriddoes not need any temperature control to ensure a precise, stable 90° phase shift. On the second diagram,one can see that the variation of the phase shift versus the wavelength is small (less than 3°). However if forsome particular application one need to have a veryprecise 90° phase shift, the phase tunability optionavailable on COH-X will enable to adjust it precisely for any wavelength of the OWR.

9– Application note: full tunable 90° hybrid

In order to have the possibility to control all theparameter of a COH24-X or a COH28-X, we propose ina19inches-2U rack a system where the user can adjust:

-the phase

-the insertion losses uniformity

-the skew

10 – Fiber type and connectors

For the single polarization hybrids (COH24 and COH24-X) both the Signal and Local Oscillator have PM fiber.The outputs have SM fiber.

For the dual polarization hybrids (COH28 and COH28-X) the Local Oscillator has PM fiber and the Signalhas SM fiber. The outputs have SM fiber.

For all of these products, the outputs can optionally have PM fiber with a PER of 20dB.

For COH28 and COH28-X, the Signal input can optionally have a PM fiber aligned such that the slow andfastaxes are split by the input PBS. Eventually the fiber can be aligned in a different way upon customerrequest.

The devices can be proposed with any kind of connectors (FP/UPC, FC/APC, SC/PC, SC/APC, LC/PC, E2000/PC,E2000/APC).

11 – Wavelength range

Both COH-X can be proposed at different wavelengthrange, from the visible to the IR. The device willbeoperational on a wavelength range of a tens of nanometers centered on the wavelength required by customer. Products at 1300nm, 1064nm and 800nm havealready successfully been manufactured.

12 – Custom product

Thanks to our free-space technology we can easily customize most of our product and so it is for the 90°optical hybrids. Customer can feel free to ask forany customization they need. We will examine the request and do our best to have a positive answer.

13 – Package layout

Table of contents

Coherent optical architectures.

Coherent mechanical packages and labelling.

Phase shift stabilityin temperature.

Phase Shift Stability Over Wave length rang.

Coherent receiver characterization methods.