Volume-phase holographic gratings benefit DWDM systems

James Arns

James Tedesco

The separation and/or recombination of numerous closely spaced wavelengths is a key task in several telecommunication applications, including optical spectrum analysis (OSA), wavelength-division multiplexing/demultiplexing (WDM), and optical add/drop multiplexing (OADM). There are several technologies already available on the market for performing these functions, all of which involve various trade-offs in cost, performance, and practical implementation. Furthermore, each must overcome technological hurdles to remain useful as optical networks move toward larger channel counts, which involve even more closely spaced wavelengths.

Recently, a novel dispersive component utilizing a volume-phase holographic grating was introduced for OSA applications. Such components provide the performance necessary for current optical networks and are well positioned to meet the needs of next-generation dense WDM systems.

Holographic grating technology

Diffraction gratings are well-known optical elements used in a wide variety of industrial and scientific applications. Traditional surface relief diffraction gratings consist of a series of closely spaced grooves on a glass or plastic substrate. When light of multiple wavelengths is incident on a grating, each wavelength is transmitted (or reflected) at a different angle, thereby allowing simple separation of the constituent wavelengths.

Surface gratings are typically produced in quantity using replication. Replicas are made by pressing a deformable, curable substance against the original "master" grating to transfer the pattern of surface relief. The ability to produce many replicas from a single master reduces the cost of individual gratings.

Surface relief gratings are relatively fragile-any contamination of, or contact with, the diffractive surface during fabrication, assembly, or use may seriously degrade performance. Surface gratings generally have a high sensitivity to input-polarization state and a spectral response that is not flat.

The volume-phase holographic (VPH) grating effectively addresses these issues. To produce a VPH grating, an optical substrate is coated with a layer of dichromated gelatin from a few to many microns in thickness. This film is exposed to an interference pattern produced by combining two mutually coherent laser beams. The exposure produces a slight, typically sinusoidal variation or modulation in the index of refraction in the material. This index variation occurs throughout the entire volume of the film, not just at the surface. After the grating has been processed to obtain high efficiency, it is laminated to a glass cover (see Fig. 1).

Because a volume grating is optically thick, the efficiency profile of the imaged light is governed by Bragg diffraction. The light path at the Bragg condition through a transmission VPH grating having fringes orthogonal to the grating surface is shown in the figure.

The VPH grating offers numerous practical and performance advantages over conventional surface relief gratings. Encapsulation between two glass substrates protects it from the environment and handling, and also enables it to be anti-reflection coated to minimize reflection-insertion loss (see Fig. 2). In addition, low polarization sensitivities are possible with both low- and high-dispersion transmission gratings. Since each manufactured grating is an optically recorded original, there are no grating replication errors and existing manufacturing processes are capable of economically producing components that approach the theoretical design parameters. Finally, novel complex grating structures can be produced to accommodate packaging constraints or improve optical performance.

Optical spectrum analysis

On-line optical-spectrum analyzers (or optical-channel analyzers) are diagnostic tools used to monitor WDM systems in the field. The OSA separates each channel from the others, so that its wavelength and signal strength can be individually monitored. To accomplish this, the system must have adequate spectral resolution and low inter-channel crosstalk. Currently, WDM channel spacing is in the 50- to 400-GHz range, but ultimately, DWDM systems will move to a 25-GHz channel spacing. Ideally, the system also should have low polarization-dependent losses (PDL), so that polarization changes do not falsely register as variations in signal amplitude. Unlike most telecom signal routing applications, low insertion loss is not a significant factor for OSAs.

The UltraSpec-C is specifically designed to achieve the high dispersion necessary to separate signals of closely spaced channels and consists of a VPH grating sandwiched between two precision right angle prisms (see Fig. 3). This grating/prism combination is sometimes called a "grism." In this device the input signal is incident at 45° to the first prism face, and nominally exits in the direction opposite to the input. The grism would be mated with appropriate imaging optics and a multi-channel detector to produce a complete OSA optical assembly.

This grism configuration offers several distinct advantages for OSA uses. First, it provides the very high internal angle of incidence necessary to satisfy the Bragg condition for a 1550-nm center wavelength on a high-dispersion 1852-lines/mm grating. In addition, the high angle of incidence increases the beam cross section on the grating, which enables high spectral resolution. This specific design delivers a dispersion of up to 0.67°/nm at the high wavelength end of its spectral bandwidth and a diffraction-limited resolution of 12.5 GHz across the C-band. Other geometries, including a larger-aperture version, can provide even greater resolution. Thus, relatively little incremental cost per channel is expected to migrate grism-based OSA designs from current standards to higher channel counts.

Another benefit of the VPH grating technology utilized here is that it can be optimized for polarization insensitivity. In the particular case of the UltraSpec-C component, the high-dispersion grating is optimized for polarization insensitivity, and achieves a PDL of 0.2 dB (typical). Insertion loss, a less important factor in OSA performance, is 4.5 dB (typical).

In terms of practical characteristics, the grism is mechanically compact (30 × 30 × 10 mm) and rugged, a distinct advantage for OSA packaging designs. It has an operating temperature range of 0°C to 70°C, and can be handled easily during assembly because the VPH grating is entirely encased.

WDMs and OADMs

With low per-channel cost and the ability to support channel spacing of less than 25 GHz, the VPH grating also holds great promise for use as a component in WDMs and OADMs. In this application, it competes with discrete filters, fiber Bragg gratings (FBGs), arrayed waveguides, interferometric fiber interleavers, and conventional surface-relief gratings. Unlike OSAs, the performance requirement for WDMs and OADMs also includes low insertion loss.

The VPH gratings can be configured to meet WDM and OADM application requirements. Fiber-spacing concentrators incorporated into the design allow small packages with low fiber-insertion loss and good channel separation. Multiple passes through a high-efficiency VPH grating provide high throughput with very high dispersion. Combining these with other design parameters creates new ways to incorporate VPH grating technology into WDM and OADM devices.

The ability to separate or combine numerous, narrowly spaced channels in a high-density DWDM spectrum with a single optical component makes the volume-phase holographic (VPH) grating an attractive alternative to many competing technologies. The VPH grating promises to reduce overall system component count and complexity, and its high spectral resolution positions it well to meet the needs of both existing and future DWDM systems.

FIGURE 3. Sandwiching a VPH grating between two precision right-angle prisms allows signals in closely spaced channels to be separated.