An inexpensive all Optical Signal to Noise Ratio (OSNR) Monitor

Ref: 11016
The invention allows superior control of networks by providing information about the operational state of key network elements. It relies upon a nonlinear process to produce a signal whose average amplitude is a unique function of the optical signal to noise.

Key advantages
  • Low cost - avoids expensive high bandwidth detector
  • Operates at bit rates in excess of 100 Gb/s
  • Highly sensitive over a broad range of OSNR. Remarkably, the monitor is effective at OSNR levels greater than 40dB when it is operated close to the Brilluoin threshold
  • Robust to system impairments such as chromatic dispersion (CD) and polarisation mode  dispersion (PMD) 

Background

The modern optical networks are susceptible of signal degradations, due to corrupting factors such as the amplified spontaneous emission (ASE) noise. Measurement of OSNR at a range of different locations in an optical system provides important information about the operational state of optical amplifiers and other key network elements. The more OSNR monitors, the better the control of the network. However, widespread deployment of conventional OSNR monitors (which convert the optical signal to an electrical one then directly measure the waveform) is expensive because the price of the optical detector becomes prohibitive at network speeds much beyond 40 Gb/s.

The standard technique to determine OSNR is to measure the ASE power levels adjacent to the signal and interpolate into the signal’s bandwidth. However, such measurements can be inaccurate because of the optical filtering and routing. A direct measurement of in-band OSNR is preferred. As a result, several all-optical OSNR monitoring schemes that involve polarisation-nulling, nonlinear power transfer functions, electrical carrier-tonoise monitoring and semiconductor optical amplifiers have been shown. However, some of these monitoring devices are limited in their sensitivity to changes in OSNR. Furthermore, these devices are often highly sensitive to variation of other signal parameters including chromatic dispersion and polarisation mode dispersion.

The invention

This invention in effect provides a low frequency signal whose amplitude is uniquely (but not linearly) related to the optical signal to noise ratio, allowing a low bandwidth, inexpensive optical detector to be used. Figure 1 shows the schematic view of the monitor for measuring in a signal from an optical network.

The device uses a tapped signal from the optical network and monitors back-reflected power from high

The device uses a tapped signal from the optical network and monitors back-reflected power from highly non-linear fiber (HNLF).

The invention relies upon the deliberate excitation of stimulated brillouin scattering (SBS) in a length of highly nonlinear fibre into which a small fraction of the network signal has been "tapped". The amplitude of the SBS signal is critically dependent on noise and hence on the OSNR. In this way, the power measured by a low-bandwidth optical detector can be directly correlated with the OSNR in the high bit rate signal.
It was demonstrated that the device is highly sensitive to OSNR levels between 7-30dB (see Figure 2). Even greater level of sensitivity (>40 dB) was achieved when the monitor is operated close to the Brilluoin threshold. Additionally, the invented OSNR monitor has an advantage in that it is insensitive to chromatic dispersion (CD) and polarisation mode dispersion (PMD).

OSNR monitoring curves of the device for 1541.3nm 40Gbls NRZ signal at three operating points (OP).

OSNR monitoring curves of the device for 1541.3nm 40Gbls NRZ signal at three operating points (OP). B) Monitoring curve when device is operated near Brillouin threshold showing appreciable sensitivity to high OSNR.

Applications

The principal area of application is next generation optical networks (both metro and long haul) where per- channel bit rates exceeding 160 Gb/s are expected within ten years.

Commercial opportunity

The global fiber optic test equipment market is valued in hundreds of millions of dollars, and is driven by increased production of fiber optic cables and by demand for upgrading existing optical networks. Sydnovate is seeking industry partners interested in licensing, collaboration and development of this OSNR technology. The technology is protected by pending US patent application.

Principal inventors

Professor Benjamin Eggleton, Dr Mark Pelusi, Timothy Iredale

Publications