Free-space diffraction gratings allow denser channel spacing

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Andrew Sappey and Pei Huang

Denser WDM systems will be needed to meet metro and regional demands. The authors argue that muxes and demuxes based on free-space diffraction gratings overcome limitations of AWG devices because of their passive operation, high performance, scalability, and low cost.

As network architects seek to apply DWDM to regional and metro networks, high-channel-count, cost-effective multiplexers/demultiplexers (muxes/demuxeses) are needed. Conventional filters made of dielectric thin films and fiber Bragg gratings are not suited to high-channel-count muxes/demuxeses because they filter light in a serial manner and so must be used in combination with other technologies such as interleavers and circulators, which can cause high insertion loss and increased system costs.

With advances in waveguide photonic integrated circuits, muxes/demuxes based on planar arrayed waveguide gratings (AWGs) and etched echelle gratings have potential use in DWDM networks. These devices process optical signals in a parallel fashion, which is preferred for high-channel-capacity networks. Yet their promise of large-volume manufacturability is hindered by typically low yields and lesser performance characteristics, including high insertion loss, low channel isolation, large channel-to-channel variations, and the need for active temperature stabilization.

Free-space diffraction-grating (FSDG) muxes/demuxes, which employ bulk diffraction gratings and discrete optical components to carry out parallel light processing, can address many of the performance weaknesses of planar AWG devices.

GRATING TECHNOLOGY EVOLVES
Diffraction gratings separate the spectrum of light from a multiplexed optical fiber into its constituent wavelengths in a parallel fashion. Light is directed to individual fibers simultaneously, rather than being sequentially filtered in a cascading manner. As a result, the technology yields increased channel capacity while eliminating the need for the additional optical elements required by thin-film filters and fiber Bragg gratings.

Bulk diffraction gratings have been used for spectroscopy and display applications for more than 150 years, and the idea of using grating technology for telecommunications applications is not new. Gratings were used for the first laboratory demonstrations of coarse WDM technology. However, because of high cost and various technical issues—including polarization-dependent loss (PDL), narrow and Gaussian passband, thermal drift, and high susceptibility to vibration—grating technology was not deployed in optical networks in favor of thin-film filter technology. For the low channel counts being deployed at the time (2, 4, or 8 channels), this decision was a good one. Advances in optical design, component fabrication, optomechanical packaging, and manufacturing automation now provide solutions to the issues traditionally associated with FSDG muxes/demuxes.

A lens in a free-space configuration collimates multiplexed light from an input fiber (see Fig. 1). This lens directs the multiplexed light, again in free space, to the diffraction grating. Assuming that Fiber 1 is the multiplexed fiber, the beams diffracted at various angles (because of the difference in wavelength) will be focused into various fibers labeled A, B, C, and so on, for their respective wavelengths.

Many different types of gratings are available, including classically ruled, surface-relief holographic, acousto-optic, and volume holographic gratings. Each type has advantages and disadvantages, but all operate on the same physical principle—induced phase shift and interference. These methods all produce phase shifts more accurately than an AWG, which also imparts a phase shift. This allows diffraction gratings to have better crosstalk and channel-isolation specifications than an AWG.

The amount of angular separation between the individual wavelengths or channels provided by the grating should be maximized in order to minimize the size of the overall device. Two methods involving diffraction-grating technologies are used to achieve this goal. One technique uses gratings with a relatively high groove or line density (perhaps 1000 lines/mm) operated in their first diffraction order to achieve high angular dispersion. Holographic gratings are limited in operation to first order and may, in fact, be used in a double-pass configuration in which the light diffracts from the grating twice to achieve the requisite angular separation between channels.

More recent designs have utilized a different type of grating called an echelle (French for "ladder"). Echelle gratings typically have a lower line density and are used in higher diffraction orders. Echelle gratings have the potential to provide high efficiency, high angular dispersion, and low polarization dependence, provided they are intelligently incorporated into the DWDM device design. Polarization dependence can be better controlled because the groove spacing can be significantly larger than the wavelength of light. Other techniques to reduce the polarization sensitivity of gratings include polarization diversity and polarization scrambling.

Diffraction gratings can be configured to produce a completely passive device, requiring no thermal-compensation elements, which network architects favor because the probability of device failure is significantly reduced. FSDG muxes/demuxes can deliver essentially no impact or vibration sensitivity when proper isolation design techniques are employed.

Although AWG devices have become more common in optical communications networks, their strong temperature dependence requires thermal regulation in the form of an active device that could ultimately fail. Passive thermally compensated AWGs have been designed, but these typically pay a large penalty in terms of other performance parameters, such as insertion loss. This is not the case with properly designed FSDG muxes/demuxes. Because no active components are needed in diffraction-grating systems, a more cost-effective and reliable solution can be delivered.

PERFORMANCE CHARACTERISTICS
Bulk gratings, the essential component in FSDG-based muxes/demuxes, can be manufactured with high precision in high volumes, enabling product repeatability and consistently high performance characteristics. With careful grating design and fabrication, high diffraction efficiency and low PDL can be achieved uniformly across the entire telecommunication band. With the supporting optics and optomechanical packaging, FSDG muxes/demuxes typically exhibit low insertion loss with small channel-to-channel variations and high cross-channel isolation.

The low insertion loss demonstrated by FSDG muxes/demuxes is primarily due to efficient fiber-to-space coupling and low grating-diffraction loss. Free-space optics permit low aberration and a high degree of mode overlapping and, therefore, provide efficient fiber coupling. The Gaussian mode that propagates naturally in the fiber core is the natural mode in free space and eliminates the need for complex mode-matching structures that must be etched into the waveguides of the most efficient AWGs. Grating efficiencies above 90% in a particular diffraction order have been achieved. Furthermore, low insertion loss can be attained more uniformly across a much wider band of wavelengths than with AWGs. Insertion loss as low as 3 dB with less than 0.5 dB across the entire 35-nm C-band has been reported (see Fig. 2).

Traditional FSDG muxes/demuxes suffered from high PDL, but newer designs reduce PDL to acceptable levels. Some systems use a polarization-diversity design or polarization scrambling to achieve low PDL, but design and fabrication of the grating itself can solve issues with PDL at its root cause without adding additional components. Through the use of polarization-insensitive gratings, PDL levels of a maximum of 0.4 dB can be achieved.

Channel passband shape represents another important parameter for all types of DWDM devices. Network architects desire a wide channel passband that allows for wavelength drift of the source laser and data-signal modulation broadening, while maintaining high channel isolation. A wide passband is also needed to accommodate wavelength inaccuracies in the DWDM device caused by manufacturing and variations in temperature.

Diffractive systems provide a Gaussian passband-filter function that has very low insertion loss at the channel center. However, the Gaussian shape can cause undesired changes in insertion loss if the laser wavelength drifts or if the center of the passband shifts with changes in temperature. In addition, in most long-haul networks, the light must be multiplexed and demultiplexed many times, thus requiring muxes/demuxes to be cascaded in series. The Gaussian passband becomes unacceptable for these applications because the Gaussian filter function "collapses" when cascaded in series.

Thin-film filters, the incumbent DWDM technology, produce a flat-top passband that does not suffer from the disadvantages of the Gaussian function but have somewhat higher insertion loss. Flat-top devices are much less sensitive to changes in insertion loss due to laser wavelength drift or thermal drift. In addition, the passband does not collapse when cascaded in series. Free-space optics provide the capability and flexibility to achieve flat-top passbands with adequate bandwidth and some additional insertion loss (1.5 to 2.0 dB).

Andrew Sappey is chief technology officer and a co-founder, and Pei Huang is director of focused R&D at Zolo Technologies, 410 S. Arthur Ave., Louisville, CO 80027. Andrew Sappey can be reached at asappey@zolotech.com.

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