Wavelength lockers make fixed and tunable lasers precise

Hooman Shakouri

As WDM channel density increases, fixed and tunable lasers must maintain greater accuracy over their operational lifetimes. Integrated wavelength locking based on a temperature-controlled etalon-based wavelength locker enables passive alignment while minimizing the size of transmitter packages.Recent advances in DWDM system designs have led to a significant increase in the number of wavelengths on a single fiber. Equipment designs with up to 160 wavelengths or channels are available. Currently, 100-GHz channel spacing is widely used in field equipment, with 50-GHz and 25-GHz spacing in lab trials. Spacing of 25-GHz channel translates to an accuracy of ±0.02 nm (20 pm) over the operational life of the device and the environment.

With such narrow channel spacing requirements, maintaining laser output wavelength with precisions at ±1 GHz (8 pm) at the predefined ITU frequency is a challenge to conventional locker schemes. Reliable stand-alone source lasers with such accuracy are not readily available in production quantities. The aging of the semiconductor materials, fabrication tolerances, and current source inaccuracies in tunable laser transmitters are only a few causes of the inadequate stability of laser sources. The result is a growing need for wavelength lockers (see Fig. 1).

One conventional method of monitoring wavelengths is to use a wave meter with a Michelson interferometer. This type of instrument is large and expensive, and the time to make a single measurement is relatively lengthy. Dense WDM system designers demand a wavelength locker at a lower cost that is capable of fast measurements, and that has a small enough size to fit inside the laser package. Such characteristics can be found in Fabry-Perot etalon-based wavelength lockers (FPWL).

Etalon-based wavelength lockers are extremely accurate, with the reliability expected of a solid-state device. They are capable of multiwavelength locking with one device that covers all ITU channels in the S-, C-, and L-bands. Moreover, their ability to monitor optical power as well as transmitted wavelength differentiates them from competitive technologies.

ETALON PERFORMANCEIn a typical wavelength-locking configuration the laser is normally tapped via a 5/95 coupler after the modulator and then fed directly into the locker. The transmitter controller monitors the signals from the two reference and etalon photodiode currents from the locker, and then adjusts the laser wavelength and the optical power accordingly (see Fig. 2).

The structural design of a Fabry-Perot etalon-based wavelength locker typically includes a beamsplitter, an etalon, a reference photodiode, and an etalon photodiode. The reference detector measures the laser output directly (after splitter) and the other photodetector measures the transmission through the etalon. The coupling ratio of the splitter in the wavelength locker is designed such that at each exact ITU channel, the optical power levels falling on the two photodiodes are equal. As the laser frequency changes while the etalon detector photocurrent varies periodically, the ratio of the two etalon and reference photodetector currents remains constant at the lock point. Therefore, by monitoring the change in the relationship of the two photocurrents, the wavelength of the laser can be monitored and stabilized. In addition, in this configuration the reference photodiode can be used as an optical-power monitor for the laser.

A Fabry-Perot etalon is capable of working as a wavelength locker over the entire range of ITU channels in the S-, C-, and L-bands with great accuracy because it acts as a comb filter with peaks at 25-, 50-, or 100-GHz spacing. It is essentially a passive component with a fixed optical path between two partially reflective mirrors. The periodic maxima of the wavelengths form as a result of the constructive interference at the mirror surfaces (see Fig. 3).

The transmission is defined by the Airy function:

Where:

and R is the etalon surface reflectivity, l is the transmitted wavelength, f is the index of refraction of etalon, d is the etalon thickness, and q is the angle of incidence.

The distance between maxima is defined as the free spectral range (FSR) and is the etalon peak spacing. Free spectral range is normally designed to be equivalent to system channel spacing (100, 50, or 25 GHz):

where c is the speed of light.

To obtain maximum sensitivity and capture range, the locking is achieved on either the positive or negative slope of the transmission rather than on the peak.

CHALLENGES FOR ETALON-BASED LOCKERSTwo primary types of etalons used in wavelength lockers are solid and air-spaced etalons. Air-spaced etalons are fabricated from two precision optical surfaces separated by a fixed space with air as the cavity medium. Solid etalons are, however, fabricated from a single optical solid substrate with flat polished surfaces and a uniform optical thickness. Although using either air-spaced or solid etalon-based wavelength lockers has its advantages over conventional designs, there are some practical issues that the optical design engineer must take into account—etalon size, active alignment requirement of the etalon, temperature sensitivity of etalon, and varying lock-point ratio, to name a few.

Integration. To effectively make use of the benefits that DWDM systems offer, network integrators are constantly exploring new ways of reducing size and cost by driving component manufacturers toward integration. This, of course, means more components in increasingly smaller packages. In the case of wavelength lockers, they must be small enough to fit in the same package with the laser. Selecting the correct etalon, therefore, is crucial. Typically, air-spaced etalons are relatively large and take a large space in multicomponent-congested laser packages. As free spectral range decreases, for example, to 25 GHz, the same etalon will have to be twice as long. A solid etalon of the appropriate material is very small and has the necessary dimension to form the core of a compact locker assembly.

Active Alignment. The transmission peaks through an etalon do not naturally take place precisely onto the ITU channels (see Fig. 4). The optical path length of the etalon must be closely tuned to properly align the lock points on the etalon fringes with the desired ITU channels. One of the methods widely used to fine tune the etalon is active tilting of the etalon as shown in Fig. 4. The maximum tilt required to move the fringes in a 50-GHz channel-spaced etalon is approximately ±1.5°. Therefore, for a required accuracy of ±1 GHz, the alignment must be as accurate as ±0.06°. This process requires tilting the etalon while actively monitoring its output and maintaining a constant temperature during the operation. The technique is very costly and requires a significant amount of time and resources during assembly process. It is also not reversible once the package is sealed.

Another technique for fine tuning (shifting) the etalon fringes over the ITU frequencies is to change the index of refraction of the etalon by altering the etalon temperature. The output of the laser then can be monitored by maintaining the temperature of the etalon to a constant value. In this method the etalon is passively aligned and then tuned after the package is sealed.

Temperature insensitivity. A major benefit of direct temperature control of the etalon is the drift-free operation over operational temperature range and life of the device Extreme wavelength stability is very crucial in almost all long-haul systems. In long- and ultralong-haul applications, system designers require laser output wavelengths to be as stable as ±1 GHz over the operating temperature range and life of the device. Standard air-spaced etalons are normally stable in an environment where the temperature varies. A standard solid etalon, however, is very temperature sensitive and can drift anywhere from 1.5 to 10 GHz per °C depending on the etalon material. In a temperature-controlled wavelength locker, however, since the etalon temperature is normally kept above the operating temperature of the environment, the locker effectively becomes immune to any ambient thermal changes (see Fig. 5). To attain wavelength stabilization within ±1 GHz, the etalon temperature must be maintained at a constant value via a closed-loop temperature-monitoring scheme with an accuracy of ±0.67°C.

Constant calibration/lock point ratio. Because of tolerances in material and assembly techniques, the lock-point ratio between the two photocurrents is not always one. During calibration of wavelength lockers, a specific ratio is determined for each locker and it is usually programmed in the firmware controller of the transmitter. For accurate wavelength stabilization, in standard solid and air-spaced etalon-based wavelength lockers, the etalon temperature must be stabilized and/or monitored. For these types of lockers, in which the lock-point ratio varies as the ambient temperature changes, adjustments to the lock-point ratio must be entered into the firmware controller of the transmitter as a correction factor. A temperature-controlled etalon-based wavelength locker is insensitive to ambient temperature variations and the frequent lock-point ratio adjustments in control firmware are eliminated.

Channel spacing. The natural evolution of DWDM systems means ever greater channel density in the fiber at faster data rates. To minimize dispersion and considering the instability of lasers, channel spacing in today's systems can only be as wide as 100 GHz for 10-Gbit/s modules and as narrow as 25 GHz for 2.5-Gbit/s modules. Systems in development are incorporating more channels at higher speeds—as narrow as 12.5 GHz at 2.5 Gbit/s and 50-GHz at 10 Gbit/s. The availability of solid etalons with various material and indexes of refraction will help enable the development of networks with channel spacing as narrow as 6.25 GHz.

With the rapid growth in the number of channels used in DWDM systems, stabilizing the laser output at a precise wavelength has become progressively more important. Choosing the appropriate type of etalon-based wavelength locker is crucial to meet this need. The ability to integrate the locker with other components, the effect on capital equipment and product costs, and design flexibility will determine the exact selection of etalon-based technologies for wavelength locking.

Hooman Shakouri is senior product and marketing manager for Tunable Photonics, 2400 Lincoln Avenue, Altadena, CA 91001. He can be reached at [email protected].