Chirped fiber Bragg gratings equalize gain

Martin Guy and François Trépanier

Gain-flattening filter technologies provide gain equalization in optical amplifiers with decreased manufacturing costs. Chirped FBGs in particular are well-suited to minimize error, while keeping undesirable effects such as PDL, PMD, and temperature dependencies low.

The successful deployment of WDM systems is directly related to the emergence of high-performance optical amplifiers. Flat-gain optical amplifiers across the whole communication bandwidth are needed to ensure proper amplification of every channel in WDM communication systems. To perform amplifier gain equalization, one of the options is to insert within the amplifier a gain-flattening filter (GFF), precisely tailored to the inverse gain curve.

Several GFF technologies can be used to perform the task of gain equalization. Among them, thin-film dielectric filters, sinusoidal filters, and fiber gratings (Bragg and long-period) are presently offered on the market. Some of the requirements for these filters are that they match as closely as possible the inverse gain curve of the individual amplifiers, introduce very low polarization dependence, minimize the gain deviation in a cascade of amplifiers, have a small package footprint, and be easily manufactured in high volume. Also, the technology that will prove to be best should be low-cost and independent of a unique technology such as a special fiber (see table).

PROVEN TECHNOLOGIESAmong the technologies available, the thin-film filter is the most proven and mature technology. Thin-film GFFs are typically based on resonant cavity design and are packaged with GRIN lens collimators. They can be used either in reflection or in transmission mode. Complex GFFs can be obtained by using a cascade of multiple simple filters or by designing a more complex single filter because, in theory, any filtering function can be obtained using an interference coating.¹ Although this option reduces the packaging complexity, it suffers from a complex fabrication process (hundreds of coating layers) and is not easily adaptable to new gain profiles. Moreover, as thin-film GFF is a bulk technology, insertion loss can be in excess of 1 dB for complex gain profile.

Sinusoidal filters such as Mach-Zehnder devices can be made using different technologies such as fused biconic tapers, planar lightwave circuits, or micro-optics. As the name implies, these filters have a sinusoidal function in which the free spectral range is adjusted to cover the desired operating window of the amplifier. Fourier analysis of the erbium-doped-fiber-amplifier (EDFA) gain spectra enables determination of numbers and depths for filters needed. To cover the complete EDFA C band, three to five sinusoidal filters are usually required. This large number of filters greatly increases the packaging size and complexity. Also, this technology has yet to be proven for high-volume field deployment.

Fiber-grating-based GFFs can be divided into two categories: long-period and Bragg gratings. In long-period gratings (LPG), a periodic structure much greater than the signal wavelength (on the order of 200 to 400 µm) is formed in the core of a photosensitive fiber.² In this type of structure, light interacting with the grating is coupled into forward-propagating cladding modes where it is rapidly attenuated because of absorption and scattering. This wavelength-selective lossy device has the main advantage of having very low back-reflection, thus avoiding the use of optical isolators when incorporated within the amplifiers. As with sinusoidal filters, however, many gratings are required to cover the full EDFA bandwidth, increasing the manufacturing processes required. Moreover, the operation wavelength of an unpackaged long-period grating is very sensitive to temperature drift by a factor at least five times larger (~50 pm/°C) than a Bragg grating. To reduce this temperature sensitivity, writing the LPG in a special fiber may be required. In addition to temperature sensitivity, LPGs are also very sensitive to bending losses. All these combined factors add complexity to the athermal packaging design.

FIGURE 2. Effect of cascading five CFBG-based GFFs. The top graph presents the five individual error functions accurate within ±0.25 dB (the insert shows the target profile), while the bottom example averages the sum of the five individual error functions, showing an accuracy of ±0.1 dB.

Spectrally designed Bragg gratings (tilted or chirped gratings) are other options that are gaining more and more popularity as gain-flattened optical amplifiers are deployed. For tilted Bragg gratings, the interference fringes are slanted while the grating is being written. Most of the guided mode interacting with the grating is coupled into radiation modes in a counter-propagating direction. Tilted Bragg gratings used as GFFs thus exhibit the main advantage of low residual back-reflection level.³ If this level is low enough, it avoids the use of an optical isolator when the filter is incorporated in the amplifier. For complex equalization gain profile, multiple tilted Bragg gratings are required. Although these types of gratings have shown adequate gain-equalization performances, they may be difficult to reproduce in a controlled manner.

CHIRPED GRATINGS
GFFs using chirped fiber Bragg gratings (CFBG) have been successfully demonstrated and may prove to be an optimal choice for gain equalization.4 These reflective filters are fabricated by changing the period of the untilted interference fringes along the length of the grating while adjusting the refractive index modulation to match the required transmission loss at a specific wavelength. For gain-flattening applications, these filters are usually used in a transmission mode. These all-fiber filters can be fabricated in compatible standard single-mode fiber, leading to very low splicing losses with conventional fiber such as SMF-28. Polarization-maintaining fiber such as PANDA or elliptical core type can also be used to write the grating when a flat-gain polarization-maintaining EDFA is to be designed. Moreover, being an all-fiber technology, out-of-band insertion loss for these devices is kept below 0.1 dB and is mainly introduced by the UV-induced index change in the fiber obtained during the writing process. With an appropriate annealing process, these devices can be made very stable, leading to a service lifetime exceeding 25 years.5 In comparison to most other gain-flattening solutions, only one filter is required to cover a very large optical bandwidth (>35 nm), which translates into a very small package footprint.

IMPROVED FABRICATION
It is important to mention that the process used to fabricate these types of GFFs is very flexible and can be adjusted very rapidly to new gain profiles. For the optical amplifier designer, this is a very cost-effective and attractive solution because it can lead to very short prototype-development time. In a manufacturing environment, automated processes dedicated to high-volume production of these GFFs are already available to sustain the actual gain-flattened-optical-amplifier market growth.

One of the main features of these filters is the very small error function that can be obtained relative to the inverse gain curve of the optical amplifiers over a wide optical bandwidth. This small error function is extremely important when GFFs are used in a cascade of amplifiers. This function avoids large signal-to-noise discrepancy among the WDM channels that may lead to significant sensitivity penalty at the receiver (see Fig. 1).

COMBINING COMPONENTS
Because CFBG-based GFFs are reflective filters usually used in transmission, they must be combined either with isolators or circulators for efficient operation within an optical amplifier. The combination ensures that no significant gain or noise-figure degradation is observed. One may argue that the use of an additional optical isolator is detrimental to the net gain of the amplifier. However, it has been shown that when the filter is incorporated in the middle of a two-stage amplifier, the losses induced by the additional optical isolator are compensated in the second stage of the amplifier.⁴

When designing an optical amplifier, the polarization-dependent loss (PDL) and polarization-mode dispersion (PMD) of the numerous optical components used in the amplifier must be kept to a minimum. This is particularly important when optical amplifiers are used in a cascade configuration with a bit rate larger than 10 Gbit/s. Typically, when the manufacturing processes of the CFBGs are well mastered, PDL is kept below 0.1 dB. Furthermore, for negligible UV-induced birefringence, typical value of PMD obtained for these types of GFFs is smaller than 0.1 ps.

Another important aspect that must be considered when inserting GFFs in optical amplifiers used in a cascade configuration is the cumulative effect of the error function of the individual filters. In the case where individual GFFs incorporated in the optical amplifiers of a chain are similar, the error will accumulate in a linear fashion.

For example, let's assume that GFFs (manufactured so they all present exactly the same error function of ±0.25 dB) are incorporated in five optical amplifiers to be cascaded. In the worst-case situation, at the output of the amplifier chain, the largest power deviation between the strongest and the weakest channels will be 2.5 dB. For a long cascade of amplifiers (in submarine systems, it is not unusual to cascade 100 to 200 amplifiers), this can lead to large unacceptable bit-error-rate discrepancy among the channels. On the other hand, CFBG-based GFFs can be manufactured in a way such that the error function will not accumulate in a linear fashion when the filters are cascaded, thereby greatly reducing the power discrepancy among the strongest and weakest channels in a long cascade of amplifiers (see Fig. 2).

Although many GFF technologies are commercially available, it is clear that the best GFF solution will be dictated by the optical-amplifier manufacturers' stringent requirements. Optimal match with the inverse gain curve is certainly of prime importance. All-fiber technology such as CFBGs is perfectly suited to achieve minimal error function, while keeping undesirable effects such as PDL, PMD, and temperature dependencies to a minimum. The CFBG manufacturing process is flexible enough that new GFF profiles can be rapidly available for quick amplifier performance adjustments. But more importantly, high-quality CFBGs can be manufactured in volume to meet the constantly increasing market demand.

REFERENCES

M. Tilsch et al., NFOEC (1999).
A. M. Vengsarkar et al., J. Lightwave Tech. 14, 1 (1996).
R. Kashyap, R. Wyatt, and R. J. Campbell, Elec. Lett. 29, 2 (1993).
M. Rochette et al., Phot. Tech. Lett. 11, 5 (1999).
H. Singh, Lasers and Optronics (June 1997).

Martin Guy is chief technology officer and François Trépanier is R&D program manager at TeraXion, 360 Franquet Suite 20, Sainte-Foy, Que., Canada, G1P 4N3. Both authors can be reached at 877-658-8372 or [email protected]