Jill D. Berger, Yongwei Zhang, John D. Grade, Howard Lee, Stephen Hrinya, Hal Jerman, Al Fennema, Alex Tselikov and Doug Anthon.
Iolon, Inc., 1870 Lundy Ave., San Jose, CA 95131
Widely tunable semiconductor lasers have many potential applications in dense wavelength division multiplexing (DWDM) networks, including wavelength conversion, optical routing and multi-wavelength sparing. Among the many techniques currently being investigated to produce such lasers [1-5], external cavity diode lasers (ECL) offer significant advantages, including wide tuning ranges, high output power, narrow linewidths with good side mode suppression and accurate wavelength control. Such devices are widely used in test equipment, but the size and complexity of the optomechanical assemblies have limited their use in optical networks . The use of silicon MEMS to perform the mechanical tuning functions makes it possible to greatly reduce the size and complexity of the devices and to match the performance of a conventional ECL in a small, robust package, suitable for use in optical networks.
Figure 1. Photograph and schematic of the MEM-ECL.
In the MEM-ECL in Fig. 1, a conventional InGaAsP/InP multiple-quantum-well laser is placed in a grazing incidence tunable resonator . The output from the antireflection-coated laser facet (R = 10-4) is collimated by a diffraction-limited silicon microlens, and split almost equally by the grazing-incidence grating into the directly reflected, zero-order output beam, and the first-order diffracted beam that is reflected by the mirror back to the laser diode. The resulting intra-cavity filter has a 3 dB passband comparable to the 25 GHz mode spacing of the resonator, and forces the laser to operate on the cavity mode closest to the center of the filter. The mirror is mounted on an electrostatic comb-drive actuator where an applied voltage rotates the mirror about a virtual pivot, synchronously tuning the passband and the cavity phase to give continuous tuning without mode hops. Adding an independent phase adjustment actuator can optimize performance by centering the laser frequency in the filter passband. The 85-micrometer thick MEMS actuator is fabricated from single-crystal silicon using deep reactive ion etching (DRIE) to produce a low-fatigue flexural structure with excellent actuator force and out-of-plane stiffness . The angular range of the actuator determines the tuning range, and a variety of ± 140 V actuators with ranges up to ± 2.5 degrees have been used to tune over almost half the 80 nm gain bandwidth of the laser diode. The MEM-ECL is assembled on a ceramic substrate, mounted to a thermoelectric cooler and coupled through a 30-dB isolator to polarization maintaining fiber with 70% efficiency. The laser itself fits in the 14-pin butterfly package often used for DFB packaging, and with the addition of components for wavelength locking it can be placed in a slightly larger 18-pin package.
Figure 2. MEM-ECL lasing spectra over 67, 50-GHz ITU channels with a 20 mW fiber-coupled output and side mode suppression better than 50 dB.
The series of spectra in Fig. 2 shows the 20 mW fiber-coupled output from a MEM-ECL over a 26.4 nm range covering 67, 50GHz ITU channels, with 55 dB side mode suppression. Similar devices operating in the C and L bands have been locked to 25, 50 or 100 GHz channels and have tuned over ranges up to 38 nm. The MEM-ECL has relative intensity noise (RIN) of less than -145 dB/Hz from 10 MHz to 10 GHz, polarization extinction ratio better than 20 dB, and spontaneous emission background 40 dB below the laser peak value. The fundamental, spontaneous-emission-induced laser linewidth depends on cavity length and, at less than 1 MHz, is narrower than that of a typical DFB laser. Mechanical vibrations broaden the observed time-averaged linewidth, and a homodyne measurement with a 25 ms delay gave a time-averaged 3 dB linewidth of 2 MHz. The actuator voltage determines the laser wavelength with an open-loop accuracy of 100 pm. DWDM systems with 50 GHz channel spacing typically require frequency accuracy of 2.5 GHz (20 pm), which is achieved by coupling a small fraction of the output to an etalon wavelength locker and using the error signal to make fine adjustments of the mirror position. With the servo enabled, the wavelength is stabilized to within 10 pm. The laser can be tuned to another channel and locked in less than 15 ms.
In summary, silicon DRIE-MEMS actuators have enabled a small form factor, low cost tunable laser source ideal for many DWDM applications. The performance of the MEM-ECL meets telecommunications requirements for optical power, side mode suppression, polarization extinction ratio, relative intensity noise, and linewidth. Frequency accuracy of 2.5 GHz is obtained with closed loop control of the MEMS actuator voltage.
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