External Cavity Diode Lasers Tuned with Silicon MEMS

The last decade has seen an explosive increase in data traffic, which has driven a tremendous increase in the capacity of long-haul telecommunications networks. This capacity increase has been enabled by the use of two technologies: dense wavelength-division multiplexing (DWDM) and optical amplification. DWDM is the technique of sending multiple data streams down a single optical fiber by modulating a number of lasers, each operating at a slightly different wavelength. Current systems can utilize hundreds of optical channels per fiber, each modulated at either 2.5 Gb/s or 10 Gb/s. Optical amplification is the technique of directly amplifying the combined optical signals, typically using erbium-doped fiber amplifiers or Raman amplifiers for ultra-long reach. This allows transmission of optical signals distances as far as 3000 km without the need for conversion of the optical signals back into the electrical domain.

The current long and ultra-long haul systems are adequate for today's data traffic. However, as systems evolve there will be a need for increased flexibility in switching and routing of optical signals. This will be particularly important as carriers seek cost-effective ways of lighting the thousands of kilometers of dark fiber already in the ground. A number of technologies need to be introduced to achieve these requirements, the most critical one being tunable lasers. Tunable lasers are the key enabler of dynamic wavelength provisioning, which will allow the full economic benefit of optically transparent networks. Widely tunable semiconductor lasers have many uses in DWDM networks, including wavelength conversion, optical routing and multi-wavelength sparing.

Among the many techniques currently being investigated to produce such lasers,^1-6 external cavity diode lasers (ECLs) offer significant advantages, including wide tuning ranges, high output power, narrow line widths with good side mode suppression and accurate wavelength control. Such devices are widely used in optical test equipment, but the size and complexity of the optomechanical assemblies have limited their use in optical networks.⁴ The use of silicon MEMS to perform the mechanical tuning function makes it possible to greatly reduce the size and complexity of the devices, and combining the ECL and an etalon wavelength locker (WLL) in the same package results in a compact, robust device suitable for use in optical transport networks.

The ECL in Figure 1 combines an indium phosphide gain chip with a tunable resonator. The light from the antireflection-coated facet of the chip is collimated by a diffraction-limited silicon microlens, and split by a grating into the directly reflected, or zero-order beam, and the first-order diffracted beam. Light is diffracted from the grating at different angles depending on the wavelength of the light. Thus tuning is accomplished by arranging a mirror to reflect light at a particular angle, and thus wavelength, back to the diode gain chip, sustaining lasing in the cavity. The reflecting mirror is mounted on a MEMS actuator chip, which rotates the mirror. The grating is designed to minimize the zero-order output and to maximize the diffracted light in the optical cavity.

1. Schematic view of the packaged MEM-ECL-WLL, showing the ECL on the left with a virtual pivot MEMS actuator and the WLL and fiber coupling on the right.

The output beam from the ECL emerges from the opposite facet of the gain chip, and is collimated and coupled into an output fiber. A small portion of the output light is split off from the main beam and directed to a WLL, which is required to accurately control the wavelength and optical power of the laser light. The WLL provides electrical signals that are a function of the wavelength and power of the light. These are used by the electrical control system of the laser to accurately maintain and switch the wavelength of the laser light from one optical channel to another. The requirements for wavelength stability are quite extreme. The tunable laser controls the frequency of the light to within ±1.25 GHz over a total tuning range of more than 5000 GHz over temperature extremes and over the life of the laser, which can exceed 20 years.

The availability of high-performance MEMS actuators has opened up a new class of devices for telecommunications systems. Many of the large-array optical switches have used surface micromachining — the technique in which sensors and actuators are made from patterning thin films (such as polysilicon) on the surface of a silicon substrate and sacrificially etching another material, such as silicon dioxide, to free the mechanical structure. Most accelerometers for deploying air bags are now made in this way.

	Etch shallow cavity in carrier wafer to create future movable areas. Fusion bond device wafer to carrier: polish to final thickness. Oxidize: open contact holes; deposit and pattern pad metal. DRIE etch through device water.
2. Cross-sectional view of the fabrication process for the DRIE-MEMS actuators used to tune the laser.

The actuator fabrication process is relatively simple and requires only five masks, as shown in Figure 2. Alignment marks are first formed on the front and back surface of an oxidized carrier wafer. Shallow cavities are then plasma etched into the silicon on the front surface. These cavities will define what portions of the actuators will be free to move and which will be attached to the carrier.

A second wafer is fusion-bonded to this front surface, and then ground and polished to the desired actuator device thickness, in this case 85 µm. The alignment marks from the bottom surface are then transferred to the new top surface to allow alignment with respect to the now enclosed cavities in the device. After an oxidation, contact hole etch and metalization deposition and patterning, DRIE is performed through the thickness of the top wafer until it intersects the cavity.⁸ A photograph of an actuator with an attached mirror is shown in Figure 3.

	3. Photograph of MEMS actuator showing the retro-reflecting mirror attached to the MEMS device.

	4. Superposition of lasing spectra showing tuning and locking of a 20 mW MEM-ECL over 103 50 GHz ITU channels in the L band.

	5. Spectrum of a single locked C-band channel showing a side mode suppression of 55 dB.

The ECL has optical performance comparable to or better than that of distributed feedback (DFB) lasers currently being used in long-haul systems. The optical performance is required for longhaul systems to minimize distortion in the modulated optical system as it propagates along the fiber. One important optical property of the laser is the line width, or the apparent variation in wavelength of the output of the laser as a function of time. The fundamental, spontaneous-emission-induced line width depends on cavity length and, at less than 1 MHz, is narrower than that of a typical DFB laser. Without additional servo control, mechanical jitter of the MEMS actuator tends to broaden the observed time-averaged line width. Typically the time-averaged line width is about 2 MHz. The laser can be tuned to another channel and locked in times as short as 15 ms.

Thermal and vibrational sensitivities are important issues in a tunable laser, and are addressed by a combination of thermo-mechanical design and servo control. Mounting the ECL and WLL components on a thermo-electric cooler eliminates the majority of thermal issues. Figure 6 shows the locking performance of a typical channel over temperature. The laser frequency in Figure 6(a) deviates from the ITU channel by less than 500 MHz and exhibits no mode hops as the case temperature is varied from -10 to 75°C with the wavelength locker servo activated. Meanwhile, the laser power in Figure 6(b) deviates by less than 2 percent.

6a. The laser frequency deviates from the ITU channel by less than 500 MHz and exhibits no mode hops as the case temperature is varied from -10 to 75°C with the wavelength locker servo activated. Steps in data correspond to instrument resolution.
6b. The laser power deviates by less than two percent and exhibits no mode hops as the case temperature is varied from -10 to 75°C with the wavelength locker servo activated.

A key component of reducing the vibrational sensitivity is to minimize the vibrational response of the MEMS actuator shown in Figure 3. This type of device exhibits a mechanical resonance below 1 kHz. Careful mechanical design of the actuator is important for minimizing response to external vibration. At higher frequencies, damping due to air in the package, the inertia of the actuator itself, and conventional shock mounting techniques minimize the coupling between external vibrations and the laser.

	7. The vibrational transfer function (0 dBv = 150 MHz/G) as the laser is subjected to vibrational excitation from 10 Hz to 10 kHz with (a) the wavelength locker servo off and (b) the wavelength locker servo activated.

Silicon DRIE-MEMS actuators have enabled a small form factor, tunable laser source ideal for many DWDM applications. The performance of the MEMS-ECL meets telecommunications requirements for optical power, optical properties and line width. Frequency accuracy of ±1.25 GHz is obtained with closed-loop control of the MEMS actuator. Substantial immunity from external temperature fluctuations and vibrations can be achieved through a combination of thermo-mechanical design and servo control.