- Optical hybrid
A 90° optical hybrid is a six-port device that is used for coherent signal demodulation for either homodyne or heterodyne detection. It would mix the incoming signal with the four quadratural states associated with the reference signal in the complex-field space. The optical hybrid would then deliver the four light signals to two pairs of balanced detectors.
Why optical hybrids are needed
Since the late 1990s, the transport capacities of long haul and ultra-long haul fiber-optic communications systems have been significantly increased by the introduction of EDFA, DWDM, dispersion compensation, and FEC technologies. For fiber-optic communications systems utilizing such technologies, the universal on/off-keying (OOK) modulation format in conjunction with direct detection methods have been sufficient to address data rates up to 10 Gbit/s per channel.
Several technological advancements can economically extend the reach and data capacity beyond such legacy systems and into next-generation networks, including but not limited to: 1) adoption of a differential phase-shift keying (DPSK) modulation format, as opposed to OOK; 2) developments in optical coherent detection; and 3) progress in adaptive electrical equalization technology. In combination, these technologies can boost a signal’s robustness and spectral efficiency against noise and transmission impairments.
These advancements in optical signal technology are feasible solutions in present-day optical networking. The path for an optical coherent system has been provided by 1) the deployment of DPSK modulated systems by Tier-1 network providers; and 2) the increased computational capacity and speed of electronic digital signal processing circuits in receivers, which provides an efficient adaptive electrical equalization solution to the costly and difficult optical phase-locked loop. These advances coupled with recent introduction of six-port optical hybrids make adopting and implementing an optical coherent detection scheme economically feasible. With such advances, optical networks can begin to realize the benefits already recognized in microwave and RF transmission systems for extending capacity and repeaterless transmission distances through coherent detection.
Optical coherent systems
The commercial feasibility of a coherent system for optical signal transmission was first investigated in the late 1980s as a means to improve a receiver’s sensitivity. At the time, because optical amplifiers were not yet fully developed, optical transmission systems were limited by the attenuation in optical fibers. A large number of transmission experiments with coherent detection were carried out to demonstrate its superior receiver sensitivity, which could be improved by up to 20 dB compared with that of intensity modulation with direct detection. In addition, in contrast to existing optical direct-detection system technology, because optical coherent detection systems can also detect not only an optical signal’s amplitude but phase and polarization as well, a number of other modulation schemes were also proposed, with a focus on improving the receiver sensitivity. However, the technology did not soon gain commercial traction because the implementation and benefits of an optical coherent system could not be realized by existing systems and technologies.
The development of the Erbium-doped fiber amplifier (EDFA) in the 1990s and its rapid implementation in wavelength division multiplexing (WDM) networks nearly brought most of the research on coherent detection to a halt, as optical signals could get amplified along the optical transmission link, effectively extending the reach of the optical signal. The primary focus of the development of optical fiber transmission systems gradually shifted towards developing techniques to compensate for chromatic dispersion, which became one of the most limiting impairments in optical fibers. Eventually, many techniques were developed to overcome this limit, including dispersion compensating fiber, and the focus shifted towards increasing the amount of information that could be transmitted in an optical fiber.
Because of coherent detection systems' ability to distinguish the optical phase and polarization of light, it was suggested that they could also be used to increase optical fiber spectral efficiency, effectively allowing more data to be transmitted within the same optical bandwidth, as modulation formats that take full advantage of these extra degree of freedom could be used, rather that simply using the intensity of light. Moreover, because coherent detection allows an optical signal’s phase and polarization to be detected and therefore measured and processed, transmission impairments which previously presented challenges, can, in theory, be mitigated electronically when an optical signal is converted into the electronic domain.
As demand for higher transmission capacity systems has evolved, a method that quadruples transmission capacity to 40 Gbit/s while maintaining the transmission properties of a 10 Gbit/s non-coherent transmission system has been introduced by Nortel in the OME 6500. It uses dual-polarization, quadrature phase-shift keyed modulation of the light source, and a coherent receiver with advanced electronic compensation of path impairments. Another approach for 10 Gbit/s optical coherent transmission uses heterodyne technique for electrical compensation of the chromatic dispersion in the IF domain was introduced by Discovery Semiconductors in 2005.
Implementing a coherent detection system in optical networks requires 1) a method to stabilize frequency difference between a transmitter and receiver within close tolerances; 2) the capability to minimize or mitigate frequency chirp or other signal inhibiting noise; 3) an availability of an optical mixer to properly combine the signal and the local amplifying light source or local oscillator (LO); and 4) an ability to stabilize the relative state of polarization between the transmitter and the local oscillator. These technologies were not available in the 1990s. A further setback to the adoption and commercialization of an optical coherent system was the introduction of the EDFA, an alternative low cost solution to the sensitivity issue. Notwithstanding the myriad challenges, an optical coherent system (also referred to as Coherent Light Wave) remains a holy grail of sorts to the optical community because of its advantages over traditional detection technologies:
- An increase of receiver sensitivity by 15 to 20 dB compared to incoherent systems, there-fore, permitting longer transmission distances (up to an additional 100 km near 1.55 µm in fiber). This enhancement is particularly significant for space based laser communications where a fiber-based solution similar to the EDFA is not available.
- Compatibility with complex modulation formats such as DPSK or DQPSK.
- Concurrent detection of a light signal’s amplitude, phase and polarization allowing more detailed information to be conveyed and extracted, thereby increasing tolerance to network impairments, such as chromatic dispersion, and improving system performance.
- Better rejection of interference from adjacent channels in DWDM systems, allowing more channels to be packed within the transmission band.
- Linear transformation of a received optical signal to an electrical signal that can then be analyzed using modern DSP technology.
- Suitable for secured communications.
There is a growing economic and technical rationale for adoption of a coherent optical system now. Academic and industrial research results have demonstrated that coherent optical systems are feasible today using advanced but commercially available optical components. The six-port 90° optical hybrid device is consequently developed based on such practical requirements.
Six-port optical hybrid
Six-port hybrid devices have been used for microwave and millimeter-wave detection systems since the mid-1990s and are a key component for coherent receivers. In principle, the six-port device consists of linear dividers and combiners interconnected in such a way that four different vectorial additions of a reference signal (LO) and the signal to be detected are obtained. The levels of the four output signals are detected by balanced receivers. By applying suitable base-band signal processing algorithms, the amplitude and phase of the unknown signal can be determined.
For optical coherent detection, the six-port 90° optical hybrid would mix the incoming signal with the four quadratural states associated with the reference signal in the complex-field space. The optical hybrid would then deliver the four light signals to two pairs of balanced detectors. See Figure 1 for block diagram of a coherent receiver.
Implementation of optical hybrids
For laboratory purposes, the 90° optical hybrid has traditionally been constructed using two 50/50-beam splitters and two beam combiners, plus one 90° phase shifter (See Figure 2). These optical hybrids can be implemented using all-fiber, planar waveguide technologies or free-space technology.
A 90° optical hybrid can be based on a Michelson interferometer or Michelson interferometerstructure (See Figure 2-3). The Michelson interferometer principle has been proven and tested in free-space bulk optics and optical component manufacturing. Free-space bulk optics is used in many optical components, such as circulators, polarization beam combiners, wavelength lockers, dispersion compensators, optical interleavers and Optical DPSK demodulators, to the fiber-optic communication industry. Bulk optics based devices can be coupled to commercially available fiber collimators.
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- 90-Degree Optical Hybrid
- Coherent Optical Technologies and Applications (COTA)
- Dual polarization 90-Degree Optical Hybrid
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