Product Overview
The FCOH (Fiber Coupled 90° Optical Hybrid) is a passive, micro-optics-based device designed for coherent optical signal demodulation, supporting BPSK and QPSK formats. It operates by mixing an incoming signal (S) with a local oscillator reference signal (L). This process generates four optical outputs: S+L, S-L, S+jL, and S-jL. These outputs represent four quadrature states within the complex optical field space. The signals are then directed to two pairs of balanced photodetectors, enabling differential detection of the in-phase (I) and quadrature (Q) components. This configuration facilitates the extraction of both amplitude and relative phase information via digital signal processing. In coherent receiver systems, preserving optical phase is crucial for advanced functionalities like dispersion compensation and phase noise mitigation in the electrical domain, thereby improving system performance and enabling cost-effective compensation of optical transmission impairments.
A block diagram illustrating the integration of the 90° Optical Hybrid in a coherent receiver is provided below.
[Diagram: Block diagram showing the 90° Optical Hybrid integrated into a coherent receiver. The hybrid receives a signal (S) and a local oscillator (L), outputting four signals (M1, M2, M3, M4) to balanced photodetectors for I/Q component extraction.]Features
- Passive operation
- Compact size
- Polarization Diversified Version also available
Applications
- Optical Coherent Detection
- QPSK Demodulation
Specifications
| Parameter | Min | Typical | Max | Unit |
|---|---|---|---|---|
| Wavelength Range (C- or L-Band) | 1527 | 1567 | nm | |
| Phase Difference [1] (between M1, M2 and M3, M4) | 90 | ±5 | deg | |
| Insertion Loss [1] | S→M1 | < 8.5 | dB | |
| L→M1 | < 8.5 | dB | ||
| (without connector) | ||||
| Insertion Loss Difference [1] | between S→M1 and S→M2 | < 1.2 | dB | |
| between S→M3 and S→M4 | < 1.2 | dB | ||
| between L→M1 and L→M2 | < 1.2 | dB | ||
| between L→M3 and L→M4 | < 1.2 | dB | ||
| Optical Return Loss | > 27 | dB | ||
| Optical Path Difference (skew, between M1 and M2 and between M3 and M4) | < 1 | ps | ||
| Optical Path Difference (skew, between any other two outputs) | < 2 in fiber length | mm | ||
| Operating Temperature | 15 | 35 | °C | |
| Storage Temperature | -40 | 85 | °C | |
| Optical Power Handling (CW) | 300 | 500 | mW | |
| Fiber Type | SMF-28 with 900 µm loose tube | |||
| Connector Type | TBD | |||
Notes:
- [1]. Over the stated spectral and operating temperature ranges and all polarization states.
- [2]. Subject to change, not including collimator sleeves extending from the two adjacent sides by 21 mm.
Note: The specifications provided are for general applications with a cost-effective approach. If you need to narrow or expand the tolerance, coverage, limit, or qualifications, please contact Agiltron.
Diagrams and System Flow
Function Diagram:
[Diagram: A block diagram showing the core function of the 90° Optical Hybrid. It has two inputs labeled "Signal input (S)" and "Local oscillator input (L)". It produces four outputs labeled M1 (S+L), M2 (S-L), M3 (S+jL), and M4 (S-jL).]System Integration Diagram:
[Diagram: A system diagram illustrating the signal flow in a coherent receiver. An "Incoming Signal" and a "Reference" signal are inputs. The Incoming Signal first goes through a "Polarization Controller". Both signals then enter the "90° Optical Hybrid". The hybrid's outputs (S+R, S-R, S+jR, S-jR) are fed into two "Balanced Detector" units. Each detector's output is processed by a "TIA" (Transimpedance Amplifier), then an "ADC" (Analog-to-Digital Converter), and finally a "DSP" (Digital Signal Processor) for signal demodulation.]Mechanical Dimension
Product dimensions may change without notice. This is sometimes required for non-standard specifications.
Recommendation Control Circuit: Information regarding a recommendation control circuit is mentioned but not detailed in this section.
Ordering Information
The following table outlines the structure for ordering the Fiber Coupled 90° Optical Hybrid:
| Prefix | Type | Wavelength | Version | Fiber Cover | Fiber Length | Connector |
|---|---|---|---|---|---|---|
| FCOH- | Regular = 11 Special = 00 |
1260~1620 = B Special = 0 |
Standard = 1 Special = 0 |
900 µm tube = 3 Special = 0 |
0.25m = 1 0.5m = 2 Special = 0 |
None = 1 FC/PC = 2 FC/APC = 3 SC/PC = 4 SC/APC = 5 LC/PC = 7 Duplex LC/PC = 8 Special = 0 |
Note: PM1550 fiber works well for 1310nm.
Technical Notes
Fiber Core Alignment
The minimum attenuation for these devices depends on excellent core-to-core alignment when connectors are mated. This is crucial for shorter wavelengths with smaller fiber core diameters, as misalignment can increase loss significantly. Different vendors' connectors may not mate well with each other, especially for angled APC connectors.
Fiber Cleanliness
Fibers with smaller core diameters (less than 5 µm) require meticulous cleanliness. Contamination at fiber-fiber interfaces, combined with high optical power density, can lead to significant optical damage. Such damage typically necessitates re-polishing or replacement of the connector.
Maximum Optical Input Power
Due to their small fiber core diameters, which are critical for short wavelengths and high photon energies, the damage thresholds for these devices are substantially reduced compared to common 1550nm fiber. To prevent damage to exposed fiber end faces and internal components, the optical input power should never exceed 20 mW for wavelengths shorter than 650nm. Agiltron offers a special version designed to increase power handling by expanding the core side at the fiber ends.
