Thin-film filters are the building blocks of multiplexing devices

The basic tool of multiplexing and demultiplexing devices, thin-film filters offer accurate center wavelength, broad flattop passband, high isolation, and transparence to bit rate and data format.

Chun He, Dennis Ma, and Wei-Zhong Li

Increasing channel densites and data rates in DWDM systems mandate component performance of an unprecedented stringency. At the top of a long list of requirements for DWDM components are accurate center wavelength, broad flattop passband with a low and uniform insertion loss, high isolation from adjacent and nonadjacent channels, and transparency to bit rate and data format.

Center wavelength and passband define the central working wavelength and its tolerance for allowing a particular optical signal to pass. The center wavelength coincides with one of the International Telecommunication Union (ITU) grids, which are defined in the frequency domain by 100 GHz spacing between grids and an origin at 193.1 THz. For devices with channel spacing below 100 GHz, the center wavelengths are adapted by dividing the frequency spectrum by the amount equivalent to the channel spacing in reference to the 193.1 THz origin.

The insertion loss of a DWDM device refers to the amount of signal loss in each channel while passing through the device. Lower insertion loss reduces the waste of valuable signal power, allowing a longer transmission distance before regeneration or amplification. Insertion loss of a DWDM component varies substantially, depending on the techniques used for multiplexing or demultiplexing, and the number of channels being multiplexed. For a typical thin-film filter-based, eight-channel, 100-GHz multiplexer, the insertion loss is 4 to 5 dB.

Adjacent and nonadjacent channel isolation refers to the average power leakage of a channel signal to the neighboring two channels and to the remaining channels, respectively. Channel isolation is an important issue for DWDM components because devices based on different working principles make use of the coherent interference effect. High channel isolation can eliminate crosstalk between channels, and therefore reduce network bit-error rate (BER). The typical requirement for adjacent channel isolation is >25 dB; for nonadjacent channels it is >35 dB.

Transparency to bit rate and data format is essential to DWDM components, not only because various modulation formats and keying schemes are employed on different channels, but because the networks should be upgradeable to high bit rates to accommodate increasing data traffic.

FIGURE 2. In a thin-film filter designed for 100 GHz DWDM applications, the high and low index quarter-wave layers are Ta₂O₅ and SiO₂, respectively. The total number of layers is approximately 120, with four embedded cavity layers.

Thin-film filter-based components are widely used as high-channel count multiplexers and demultiplexers, as well as optical add/drop multiplexers (OADM). Other than merely acting as a means of directing and manipulating signals, thin-film filters are widely used for gain flattening, band splitting, C- and L-band separation, and combining amplifier-pump beams. It's worth mentioning that thin-film filter technology so far is the only practical choice for the newly emerging coarse and band WDM networks.

THIN-FILM FILTER MULTIPLEXING
Inside the thin-film filter multiplexer, multilayer thin-film filters are combined with micro-optics and fiber pigtails in a mechanical-optical assembly. In a typical three-port device, a graded index (GRIN) rod lens focuses and collimates the light from the input fiber onto a thin film filter (see Fig. 1). The thin-film filter is designed to transmit a specific wavelength channel and reflect all of the other wavelengths. The filter is aligned to the GRIN lens at a small angle to its optical axis so that the reflected wavelengths are refocused by the same GRIN lens into a second fiber core located a small distance from the input fiber on the dual fiber pigtail assembly. The transmitted wavelength is focused into the third fiber core by a second GRIN lens.

The tolerance for the optical alignment is very narrow because of the stringent requirements for DWDM components, especially regarding center wavelength and insertion loss. For achieving a >1-dB insertion loss, the lateral misalignment between the fiber pigtail and the GRIN lens must be >20 µm, while for the same requirement the axial misalignment between the fiber pigtail and the GRIN lens must be kept at >0.02°. Multichannel multiplexing and demultiplexing devices are usually assembled by cascading a series of three-port devices, using the reflected beam as the input for the next three-port device.

Each individual three-port device has a filter with a passband corresponding to a specific DWDM channel and is assembled and tested to an extremely tight specification. This modular approach eliminates the process of aligning a large number of optics with high precision, therefore increasing the manufacturing yield. The alternative could be that an entire multichannel device could fail because of one failed part, a slight alignment error between two optics, or even a filter with a wrong center wavelength. This approach also adds tremendous flexibility and scalability for use in optical add/drop multiplexers or when planning for extra channels in the future. It also helps component inventory control and standardization.

Although loss in each optical surface is engineered to be very low, cascading three-port devices add extra losses as the channel number grows. Typically, insertion loss for a three-port device is less than 0.5 dB. The loss scales up with the number of channels because of the added optical surfaces in the cascading design. Channels that transmit later during the cascading sequence suffer higher losses than earlier ones. To overcome this drawback, filters with different transmission and special cascading schemes are usually combined to even the output signal intensities among different channels, or to achieve a uniform insertion loss for all channels. For an eight-channel 100-GHz DWDM multiplexer, the insertion loss for every channel is averaged to ~5 dB.

THIN-FILM FILTER MANUFACTURING
Thin-film filters work on the fundamental interference phenomena. When a stack of transparent dielectric films is deposited on the surface of a substrate the optical properties vary with wavelength. The strongest effects occur when the layers are one-quarter of a wavelength thick. A series of quarter-wave layers of alternating high and low indices provides high reflectance and is the basis for filters with low losses.

The zone of high reflectance—called the passband or the stop band—has limited bandwidth. Beyond it, the constructive interference collapses because of the phase mismatch and the transmission becomes high. One-half wavelength layers, called cavity layers are often inserted between the quarter-wave stacks to act as reflectors.

Thin-film filters usually have a size of ~1.5 x 1.5 mm² square and are cut from a special substrate having superior thermal properties. To make the filters, 100 to 200 alternating layers of two optical materials, one with a low refractive index and the other with a high refractive index, are deposited. The commonly used materials are silica (SiO₂) as the low-index layer and tantalum pentoxide (Ta₂O₅) as the high-index layer. Both materials have been proven to provide the desired optical properties in the telecom wavelength range with high thermal stability. These materials have a high refractive index contrast, reducing the number of layer pairs required for narrow passband and low passband loss.

Quarter-wave stacks with alternating high/low refractive index filters have long been recognized as the choice for making narrow-band filters with a steep slope. However, the unprecedented requirements for DWDM applications were unattainable without the development of modern thin-film deposition techniques. Three main deposition techniques are used for DWDM applications: ion beam assisted deposition (IBAD), plasma-assisted deposition (PAD), and ion beam sputtering (IBS). Their working principles share a common element—bombarding the target materials with ion beams while they are condensing on the substrate.

The collisions between the energetic projectiles and the target molecules prevents voids from forming in the films. The lack of voids has several advantages: first, the film exhibits both high density and a high degree of physical uniformity to improve yield; second, the voids in the film tend to absorb water and other molecules, causing long-term reliability issues; and third, the refractive index is more stable from daily run-to-run deposition processes. Silica and tantalum pentoxide are known to yield dense amorphous microstructures with exceedingly smooth surfaces when energetically deposited.

The solution is to combine the thin-film filters with an interleaver. An interleaver separates a multichannel input signal into two complementary sets at twice the original channel spacing—one set contains the even number channels and the other the odd number channels (see Fig. 3). Cascading interleavers provides even wider channel separations until the channel spacing reaches the level where thin-film filters can be reliably made with high yield using current deposition technology. This combination not only resolves the exasperation of declining yield for filters with channel spacing below 50 GHz, but also alleviates the problem of increasing insertion loss caused by cascading the three-port device for high channel count device modules and adjusting filter transmission for balancing insertion loss among all the channels.

For example, a 160-channel incoming signal can first be separated by a C- and L-band separator into two signals of 80 channels each, where each of the channels is spaced 50 GHz from adjacent channels (see Fig. 4). The two signals then pass through 50-GHz interleavers where the signals are split into two subsequent signals of 40 channels each, with each channel spaced 100 GHz from adjacent channels. The signals then pass through a 100-GHz interleaver resulting in two further signal sets of 20 channels, with 200 GHz channel spacing. The first 10 channels of the signal are then demultiplexed by a 10-channel, 200-GHz filter module. The remaining 10 channels pass through an upgrade port on the 200-GHz filter module and are demultiplexed by a second 10-channel, 200-GHz filter module.

Chun He is manager of active component development, Dennis Ma is senior director of marketing, and Wei-Zhong Li is chief technology officer of Oplink Communications, 3469 North First Street, San Jose, CA 95134. Chun He can be reached at 408-382-6826 or [email protected].