Contents I. Introduction
Widely wavelength-tunable lasers are indispensable as light sources for realizing future photonic networks that employ wavelength switching, such as reconfigurable optical add–drop multiplexers (ROADM) and optical cross-connect (OXC) systems. They are also expected to act as backup sources or replacements for current point-to-point dense wavelength-division multiplexing (DWDM) systems to reduce inventory cost. Many types of widely tunable lasers such as distributed Bragg reflector (DBR)-type lasers [sampled grating DBR laser diodes (LDs), superstructure grating DBR LDs, etc.] [1]–[4], distributed feedback (DFB) laser-based arrays, and hybrid integrated lasers [semiconductor optical amplifier (SOA) with microelectromechanical system (MEMS) mirror, SOA integrated with planer lightwave circuit (PLC), etc.] [5]–[9], have already been developed. The state of the art as regards widely tunable lasers is reviewed in [10]. A tunable DFB laser array (TLA) is a promising candidate for this application because its mode characteristics are stable and there is no mode-hopping problem [11]–[16]. In most of the recently reported TLAs [13]–[16], a multimode interferometer (MMI) is used as an optical combiner because it offers compactness and ease of design. However, the MMI is inherently wavelength dependent, and the transmission characteristics depend strongly on the size of the multimode waveguide. As a result, strict control is required for waveguide fabrication. On the other hand, the funnel combiner has been reported as a small wavelength-dependent combiner, since it is based on diffraction into a slab waveguide [17]–[19]. This type of combiner was used in earlier work on DFB laser arrays [11], [12]. However, the combiner has not been optimized for a compact tunable laser, and has a relatively large device size and a low output power.
