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Optoelectronics & Communications

Tuning Across the Band

MEMS cantilevers precisely adjust cavity thickness to yield broadly tunable VCSELs.

From oemagazine May 2001
30 May 2001, SPIE Newsroom. DOI: 10.1117/2.5200105.0006

Dense wavelength division multiplexing (DWDM) is one of the most effective technologies for increasing communication bandwidth. A state-of-the-art system transmits data across 200 or more wavelengths (channels) in the 1530- to 1610-nm spectral band, providing terabit-per-second capacity over a single optical fiber. Once strictly a long-haul solution, DWDM is increasingly being deployed in metropolitan-area networks (MANs).

Bandwidth management is critical in the MAN, where the demand for signal routing and switching is intense, and the traffic pattern is bursty and less predictable. Thus, metro networks require optical systems with the capability to quickly and remotely provision, redirect, and reactivate wavelengths as traffic demands. Tunable transmitters with reliable frequency control are a vital component in the realization of reconfigurable optical networks, which pave the way for protocol-transparent services that provide wavelength on demand.

Vertical-cavity surface-emitting lasers (VCSELs) are suitable for just this application. They are economical and high performing, and they can be produced in volume. The top-emitting topology facilitates wafer-scale testing and processing and improves coupling efficiency into a single-mode fiber. A typical VCSEL consists of an active region sandwiched between two oppositely doped distributed Bragg reflectors (DBRs) that form the optical cavity. A device generally supports a single Fabry-Perot (FP) wavelength within the gain spectrum, so the FP wavelength, and not the gain peak, determines the lasing wavelength. By changing the thickness of the optical cavity, it is possible to change the FP wavelength supported by the cavity. In other words, changing the thickness of the optical cavity changes the lasing wavelength of the device.

engineering tunability

By integrating the top DBR of a VCSEL with a micro-electro-mechanical-systems (MEMS) cantilever, it is possible to precisely change the cavity thickness to achieve an electrically tunable device.1,2 This approach was first demonstrated in the 940-nm wavelength regime, but we since have extended that work to devices in the 1.5 to 1.6 µm region.

Referred to as a cantilever-VCSEL or c-VCSEL, the device consists of a bottom n-DBR, a cavity layer with an active region, and a top mirror (see figure 1). The top mirror, in turn, consists of three parts (starting from the substrate side): a p-DBR, an air gap, and a top n-DBR, which is freely suspended above the laser cavity and supported by the cantilever structure. Laser-drive current is injected through the middle contact via the p-DBR. An oxide aperture formed on an aluminum-containing layer in the p-DBR section above the cavity layer provides simultaneous current and optical confinements. The top n-DBR incorporates the contact for the tuning voltage.

Figure 1. The tunable c-VCSEL (inset) consists of a bottom n-DBR, a cavity layer with an active region, and a top mirror. The top mirror, in turn, consists of a p-DBR, an air gap, and a top n-DBR. The cantilever is 3-mm wide and 100-mm long.

The design allows the entire heterostructure of the c-VCSEL to be grown in a single step, yielding an accurate wavelength tuning range and predictable tuning characteristics. The subsequent processing steps in fabricating a c-VCSEL include cantilever formation and release step.

To tune the device, we apply a reverse bias voltage across the air gap between the top n-DBR and p-DBR. The resultant electrostatic force pulls the cantilever down toward the substrate, shortening the air gap to tune the laser output to a shorter wavelength. Because the movement is elastic, there is no hysteresis in the wavelength-tuning curve; when the voltage is removed, the cantilever returns to its original position. The tuning speed of the c-VCSEL is determined by the cantilever dimensions. The 3-µm wide, 100-µm long cantilever yields a tuning time of 1 to 10 µs, which is fast compared to other larger MEMS devices.

Based on modeling and the performance of short-wavelength c-VCSELs, these devices can achieve a continuous tuning range of better than 5% Δλ/λ. For a long-wavelength VCSEL, this is equivalent to 80 nm at 1550 nm, or the sum of both the C- and L-bands. The continuous, repeatable, and hysteresis-free tuning curve enables the transmission system to lock onto a channel/wavelength well ahead of its data transmission, a capability referred to as dark tuning (see figure 2). Dark tuning is crucial for reconfigurable metro networks, which cannot tolerate channel-to-channel interference during the tuning process. If a channel being redirected/reactivated sweeps through other channels during the tuning process, it can "blind" them or otherwise introduce data transmission errors. The c-VCSEL is an electrically pumped VCSEL suitable for high-speed direct modulation over the full tuning range. As the laser wavelength is tuned throughout the tuning range, the eye diagram remains open.

Figure 2. A plot of wavelength versus tuning voltage shows a continuous, predictable and hysterisis-free tuning characteristic.

locking in wavelength

The continuous wavelength-tuning characteristic enables the design of a universal wavelength locker that does not require individual adjustments or calibration for each laser. It also enables rapid locking, resulting in a fast usable tuning speed.

A typical c-VCSEL with closed-loop feedback locking can be tuned over multiple channels on the 100 GHz ITU grid and locked to within 2.5 GHz accuracy in approximately 200 µs (see figure 3). This usable tuning time is well within the 1-ms requirement for most applications. We expect the locking time to improve to less than 100 µs for tuning across the entire C- or L-band.

Figure 3. The measured optical frequency as a function of time shows the VCSEL wavelength being tuned.

A continuous and repeatable tuning curve further implies that a tunable transmitter that is set at a given ITU-grid spacing can be easily programmed to lock onto a denser spacing without significant hardware changes or concerns of mode hopping.

The monolithic integration of MEMS with VCSEL has led to strong performance in wavelength-tunable lasers. The electrically pumped c-VCSELs are widely tunable, can be directly modulated at high data rates, have a simple monotonic tuning curve for easy wavelength locking and dark tuning, and offer fast usable tuning speed. They can be batch-processed and tested, essential characteristics of mass manufacturability. Furthermore, they can be locked via a universal locker without custom tailoring.

Tunable VCSELs are expected to reshape the horizon of metropolitan area networks by enabling a wide range of exciting new system functionalities. These functionalities in turn will bring end-users the most economical, flexible, on-demand, broadband services. oe

References

1. E. C. Vail et al., IEEE/LEOS Annual Meeting, postdeadline paper, (1994).

2. C. J. Chang-Hasnain, IEEE Journal of Selected Topics in Quantum Electronics, 6[6] pp. 978-987, (2000).


Connie J. Chang-Hasnain, Michael Jansen, Don Davis

Connie J. Chang-Hasnain, Michael Jansen, and Don Davis are with Bandwidth9 Inc., Fremont, CA.