Coherent Optical Receiver

What is a coherent optical receiver?

Modern optical links at 400G, 800G and beyond encode data in the amplitude, phase and polarisation of light using formats such as DP-QPSK, 16QAM and 64QAM. A simple photodiode only measures intensity and cannot recover this information. A coherent receiver solves the problem by beating the signal against a local oscillator (LO) laser at nearly the same frequency.

Inside the receiver, a polarisation beam splitter separates the X and Y polarisations, and a 90° optical hybrid mixes each with the LO. Balanced photodiodes then produce four electrical outputs, XI, XQ, YI and YQ, which together carry the complete optical field. A real-time oscilloscope digitises these signals and DSP software recovers the constellation, measures error vector magnitude (EVM) and bit error rate.

Why a reference receiver matters

When developing or qualifying a coherent transmitter, engineers need a receiver whose own impairments are small and well characterised, so that measured penalties can be attributed to the device under test. The CORX is designed for exactly this role: matched channels with IQ and XY skew within ±5 ps, phase angle error within ±5°, and flat frequency response up to its rated bandwidth.

What the built-in local oscillator adds

Coherent detection needs an LO with low phase noise. The CORX integrates a CoBrite narrow linewidth tunable laser covering 1524.5–1572 nm with 25 kHz typical linewidth, so no separate LO instrument is required. Where an experiment needs an external LO, the internal laser can be bypassed and used as a general-purpose light source instead.

Choosing a bandwidth class

Receiver bandwidth should match the symbol rate of the signal. As a rule of thumb, the required analogue bandwidth is roughly half to two-thirds of the symbol rate. The CORX Class 20 suits signals up to about 32–40 GBaud, Class 40 covers 64–90 GBaud, Class 60 supports 100–128 GBaud, and Class 80 targets the next generation of 130 GBaud-plus systems. Choose the class to match your oscilloscope bandwidth as well, since the slower of the two limits the measurement.

RF Specifications by Bandwidth Class

Optical Specifications

Integrated Local Oscillator Laser

General

Four Bandwidth Classes up to 80 GHz

Choose Class 20, 40, 60 or 80 to match your symbol rate and oscilloscope. Class 60 has demonstrated 128 GBaud 16QAM (1.024 Tb/s) and 100 GBaud 64QAM (1.2 Tb/s) in ID Photonics reference tests, and Class 80 extends headroom for the next generation of coherent systems.

Built-In Narrow Linewidth Local Oscillator

An integrated CoBrite tunable laser with 25 kHz typical linewidth covers the full C-band, with fine tuning at 1 MHz resolution to align the LO precisely with the signal. No external LO laser is needed, and the internal laser can serve as a general-purpose source when an external LO is used instead.

Variable-Gain Transimpedance Amplifiers

Each of the four channels has its own TIA with user-adjustable gain and peaking, plus fast internal control to hold a defined output swing up to 600 mVpp. This makes full use of the oscilloscope’s ADC range even for weak signals and compensates for channel fading in applications such as free-space optical links.

Software-Controlled Input Attenuator

A 10 dB variable optical attenuator at the signal input optimises receiver input power and compensates for power fluctuations without adding external components to the setup.

Matched, Low-Impairment Channels

IQ and XY skew within ±5 ps, phase angle error within ±5° and a 1 MHz low-frequency cut-off keep receiver-induced penalties small, so measured EVM and BER reflect the transmitter under test rather than the receiver.

Oscilloscope-Ready RF Interface

Four single-ended 50 Ω outputs connect directly to any real-time oscilloscope through 1.85 mm (Class 20–60) or 1.00 mm (Class 80) connectors. Delay-matched RF cable sets, including hybrid 1.85 mm to 2.92 mm cables, are available as accessories.

Installation-Free Web GUI and SCPI Control

A built-in web server provides full control from any browser on a PC or tablet over USB or Ethernet. SCPI-style commands support automated test scripts in Python, MATLAB or LabVIEW.

Compact 1 HE Design, Made in Germany

The complete receiver, including LO laser and amplifiers, fits in a 19″ 1 HE chassis weighing 3 kg. ID Photonics designs and manufactures the CORX in Germany.

See the full overview of coherent optical receiver applications, or explore the uses below.

Reference Receiver for Coherent Transmitter Characterisation

Transceiver and modulator developers use the CORX as a known-good reference to measure EVM, IQ imbalance, skew and BER of coherent transmitters. Paired with an OMFT multi-format transmitter, it forms a complete coherent test bench from QPSK to 64QAM.

400G, 800G and Terabit Transmission Experiments

Telecom R&D groups use the CORX in DWDM transmission experiments for telecommunications and optical networks, evaluating new modulation formats, probabilistic shaping and fibre impairments at symbol rates beyond 120 GBaud.

Coherent DSP Development and Validation

Algorithm teams capture CORX outputs on a real-time oscilloscope to develop and validate carrier recovery, equalisation and polarisation demultiplexing algorithms on real optical data, before committing designs to silicon.

Satellite and Free-Space Optical Communication

For satellite and space communication, the per-channel variable-gain TIAs compensate for the deep power fading of free-space links, letting engineers analyse coherent signals under realistic turbulence conditions.

Defence and Secure Communication

In defence and secure communication programmes, the CORX supports R&D on coherent military and aerospace links, from waveform evaluation to receiver performance testing.

Coherent LiDAR and Optical Sensing

For coherent LiDAR and interferometric sensing, the CORX provides polarisation-diverse IQ detection with a narrow linewidth LO, recovering both phase and amplitude of the returned light.

Continuous-Variable QKD Research

Researchers in quantum key distribution use coherent IQ detection to study continuous-variable protocols and characterise phase noise in their optical setups.

University Teaching and Research Labs

Universities use the CORX, together with CoBrite tunable lasers and an optical spectrum analyser, to build coherent communication teaching labs that expose students to industry-standard measurement methods.

Official Authorised Distributor in India

United Spectrum Instruments is the authorised Indian distributor for ID Photonics, so you receive a genuine CORX backed by the manufacturer.

Help Choosing the Right Bandwidth Class

The right CORX class depends on your symbol rate, modulation format and oscilloscope bandwidth. We review your test plan and recommend the class and RF cable set that give the best result without over-specifying.

Experience with Indian Research and Industry

We work with IITs, CSIR laboratories, defence establishments, telecom companies and photonics start-ups across India and understand their procurement and documentation requirements.

Delivery and Local Technical Support

We deliver anywhere in India and provide after-sales support for setup, oscilloscope integration, SCPI automation and troubleshooting.

A Complete Coherent Test Bench

Alongside the CORX, we supply OMFT transmitters, CoBrite tunable lasers, automatic bias controllers, PM amplifiers and optical spectrum analysers, so your entire coherent test setup can come from one partner.

Contact Us

Phone: +91 93631 83748 / +91 97899 04948
Email: sales@unitedspectrum.in