Integrated packaging advances tunable lasers

High-yield integrated packaging of optical functions promises to reduce the overall cost of components, since it eliminates redundant hermetic enclosures, fiber pigtails, tap couplers, and splices while improving coupling efficiency. At the same time, full-band tunable lasers offer additional savings in logistics, inventory, and deployment for DWDM applications. That is especially true for fixed-frequency lasers and narrowband optical modulators whose performance characteristics change over wavelength. A widely tunable integrated transmitter also simplifies wavelength-provisioning applications.¹

A combination of DFB laser arrays and lithium niobate (LiNbO₃) modulators can provide these capabilities, since the laser array can be tuned over 36 nm with very good performance and LiNbO₃ modulator performance characteristics do not change significantly across more than 40 nm. A description of the key performance characteristics over the C-band are shown, illustrating how novel packaging can enable low-cost, high-yield functional integration of transmitter optics for DWDM transponder suppliers and system integrators.

Figure 1. At the heart of the tunable transmitter is an array of DFB lasers. A MEMS mirror deflects the output to the wavelength locker and modulator.

The ILM incorporates a 12-stripe DFB laser array, MEMS mirror, wavelength locker, 10-Gbit/sec LiNbO₃ modulator, variable optical attenuator, and bias/power monitor photodiode. A schematic of the device is shown in Figure 1. The tunable-laser portion of the design consists of an array of 12 DFB lasers with wavelengths every 3 nm over the C-band.² For tuning, the laser closest to the desired wavelength is turned on and the temperature tuned over a 3-nm range. A MEMS tilt mirror directs light from the selected laser in the array to the wavelength locker and modulator. A quad photodiode detects the angle of the MEMS mirror. The wavelength locker consists of a 50-GHz etalon and a photodiode on a separate temperature-controlled platform. A variable optical attenuator (VOA) is placed on the wavelength-locker platform.

An aspheric lens focuses the light into the LiNbO₃ modulator waveguide.³ A confocal optical design is used to make the device optomechanically tolerant. The modulator may include a taper at the input of the waveguide to improve coupling efficiency and reduce optomechanical sensitivity. The LiNbO₃ modulator has an integrated power monitor photodiode, which is used for modulator control. It is also used for power control feedback for the MEMS mirror control loop and for the control of a VOA.

The use of the MEMS mirror as a laser selector has several benefits:

It enables passive placement of the optical subassemblies using standard pick and place automated systems—a very-low-cost method of high-performance optical integration.
When used in conjunction with the optical power monitor, the coupling between the laser and modulator can be continuously optimized to reduce variation of output power over temperature and time.
Adjustment of the MEMS mirror can compensate for the mechanical shift imparted by the laser welding of the lens, eliminating the need for post-weld laser hammering or mechanical deformation.

Over the past decade, DFB lasers have been the dominant technology for high-performance source lasers in telecommunications. Most of the laser requirements for fiber-optic transmission have been established around DFB lasers. The key laser characteristics for high-fidelity DWDM transmission are side-mode suppression ratio (SMSR), relative intensity noise (RIN), and linewidth (DI). The typical requirements for these parameter are SMSR >40 dB, RIN <–140 dB/Hz^½, and DI >100 kHz and <10 MHz. Because the ILM uses DFB technology for the tunable laser, it meets all of these requirements.

Some of the other key characteristics for a 50-GHz DWDM metro LR-2 or long haul (LH) transmitter applications are frequency stability of ±2.5 GHz, a minimum modulated output power of 3 dBm, and modulated output power variation <±0.5 dB. In amplified DWDM networks, the output powers of the individual transmitters are varied to change the spectral power density for gain tilt control of the EDFA. That is typically accomplished using a VOA in each transmitter. The dynamic range of modulated output power is typically 15 dB. The LH version of the ILM employs a liquid-crystal-based VOA. This VOA, in conjunction with the modulator, can be used for output shuttering while the laser is tuned. It can also be used to slowly bring up or down the transmitter so the network does not experience sudden changes in the spectral power density.

LiNbO₃-based Mach-Zehnder modulators have been field deployed in telecom systems for more than 15 years. Their performance and behavior is very well understood. When used in conjunction with a properly designed receiver, a modulator with good S11, S21, extinction ratio, and chirp characteristics will produce a transmission with a link penalty of about 1 dB after ±1,200 psec/nm of dispersion, or a link penalty of <2 dB with a chirped variant after ±1600 psec/nm of dispersion.

Figure 2. The transmitter shows good performance at 10.66 Gbits/sec across its entire wavelength range.

Figure 2 shows eye-diagrams of an ILM at 10.66 Gbits/sec across and some of the key modulation parameters measured across the C-band of an ILM. The control electronics perform optimization of the bias point and the extinction ratio and control the crossing level. The tabular data shows that the key modulation parameters are not sensitive to emission frequency across the C-band.

Figure 3. The module demonstrates a consistently low bit-error rate across the C-band.

Bit-error-rate (BER) experiments enable the designer to look at the back-to-back performance of the ILM over several channels spanning the C-band. Figure 3 shows that the variation in BER over the C-band is very small and limited by test capability. These tests indicate that there is very little variability in modulator performance across the full C-band. With the use of mature laser and modulator technology, which is well-behaved across an ITU band, and the knowledge of the dispersion characteristics of the fiber plant, testing of a limited number of channels should assure performance of all channels.

It is not only possible but in many ways desirable to integrate a frequency-stabilized widely tunable DFB laser array with a 10-Gbit/sec LiNbO₃ modulator and a VOA.

Using novel packaging technology, it is possible to employ passive placement of the optical subassemblies and limit the number of active alignment steps. Preliminary testing indicates that with normal functional testing of the laser, wavelocker, modulator, and VOA subassemblies, it should be possible to minimize the amount of expensive transmission testing while assuring performance over all channels.

Tim Munks is applications and business development manager, Andy Finch is an optical-design engineer, Joseph Farina is a design engineer, Greg McBrien is a design engineering manager, and Tim Wilken is a process engineer at JDS Uniphase (Bloomfield CT). Ed Vail is director of packaging, John Heanue is director of optics, Rad Olson is senior engineer, Bardia Pezeshki is vice president of engineering, and Xiaoyu Hong is a staff engineer at Santur (Fremont, CA).