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Optoelectronics & Communications

Design enhancements that boost widely tunable laser performance

Adding layers to the gain section and optimizing butt-joint angles to reduce interface reflections can increase tunable laser output power in excess of 80%.
5 July 2006, SPIE Newsroom. DOI: 10.1117/2.1200605.0257

In the last two and a half decades, great effort and vast investments have been put into the research and development of widely-tunable lasers, whose wavelengths can be changed by the user.1–3 Tunable lasers have become critical components of next-generation telecommunication networks and systems, providing channel restoration, reconfiguration and protection.1–4

Telecommunications applications have imposed stringent requirements1–4 on tunable laser properties such as optical output power, tuning range and speed, side-mode suppression ratio, line-width, chirp, and reliability. Additional layers in the gain section can increase the laser's output power. However, butt-joint reflections at the regrowth interfaces between the sections of monolithic tunable lasers can seriously impair tunability and greatly reduce the facet output power. 2 Fortunately, these problems can be substantially reduced by adjusting the butt-joint angles. Here I offer some original design ideas for implementing these improvements in multi-section, InGaAsP/InP-based tunable lasers.

A conventional InGaAsP/InP gain section with asymmetric-cladding includes six 1%-compressively strained InGaAsP multiple quantum wells (MQWs) separated by lattice-matched 1.25Q InGaAsP barriers. The MQW stack is sandwiched between a 1.25Q InGaAsP cladding layer and graded-index, separate-confinement heterostructure layers of 1.25Q-to-1.07Q InGaAsP. An InP buffer layer and substrate lie below the cladding layer, and a single-mode ridge tops the gain section.

To achieve ultrahigh output power, the gain section design can be improved by doubling the ridge width, which can be facilitated by inserting two new layers as shown in Figure 1.2 An InP spacer layer inserted below the ridge and above the MQW stack introduces extra latitude in the setting the single-mode ridge width. In addition, a bulk-balance layer structure inserted below the MQW stack and above the InP buffer layer reduces free-carrier loss by shifting the optical power distribution to the intrinsic and n-doped sides.

Figure 1. This schematic illustration shows the layers added to a conventional gain section. GRIN-SCH: GRaded-INdex, Separate-Confinement Heterostructure; MQWs: Multiple Quantum Wells.

These ideas were modeled at a wavelength of 1.55μm with a simulated 500μm-long gain section. The effects of the tuning section's grating-distributed Bragg reflector were simulated with a 4% reflectivity anti-reflection coating on the front facet, and a 95% reflectivity coating on the back facet. Figure 2 shows that the facet output power from the improved gain section exceeds the conventional gain section output by up to about 80%.

Figure 2. At most currents, the output power of the improved gain section (solid line) is much higher than power from the conventional gain section (dashed line).

Reflections from butt joints in monolithic, widely-tunable lasers fabricated with a regrowth step can cause internal losses. The reflections are mainly from the effective index of refraction difference between the laser's active and passive sections, and from the imperfect regrowth interface between these sections. The passive sections are made from higher band-gap material that is butt-coupled to the active sections, and then grown using selective area epitaxy (see Figure 3).

Figure 3. This schematic illustration shows the typical butt-joints in a four-section tunable laser.

The problems arising from the butt-joint reflections can be reduced by adjusting the interface angles. Simulations show that the optimized butt-joint angle between the gain and tuning sections is 20° for a three-section laser (see Figure 4), while in a four-section laser it is 20° for the first butt-joint angle and -20° for the second (see Figures 5 and 6). These arrangements of butt-joint angles can greatly reduce the total adverse interface reflection in the devices, improving their performance.

Figure 4. In a typical three-section, regrowth tunable laser, butt-joint transmission (dots) and reflectivity (diamonds) depend on the interface angle (‘Angle’).

Figure 5. The total transmission (stars) and reflectivity (dots) of the whole four-section structure are shown over a range of second-interface butt-joint angles (‘Angle 2’) for a 500μm gain section.

Figure 6. The butt-joint angles are optimally arranged in this four-section tunable laser with a 500μm gain section.

Simulations have shown the merit of these novel design ideas for increasing the performance of a monolithic, multi-section, InGaAsP/InP-based widely-tunable laser. The improvements to a conventional gain section increase output power by 80%, and angled butt-joints can greatly reduce the total adverse interface reflection in multi-section tunable lasers.

Yaping Zhang
George Green Institute for Electromagnetics Research, The University of Nottingham
Nottingham, UK 
Before joining the George Green Institute for Electromagnetics Research, Yaping Zhang worked for Marconi, Caswell, UK, which later became part of Bookham Inc., where she designed ultrahigh power tunable laser epilayers and angled butt-joints for the company's commercial widely-tunable laser products. She has written three papers for SPIE conferences.