Y.A. Akulova

Tunable semiconductor lasers: a tutorial

Tunable semiconductor lasers have been listed in numerous critical technology lists for future optical communication and sensing systems. This paper summarizes a tutorial that was given at OFC 03. It includes some discussion of why tunable lasers might be beneficial, an outline of basic tuning mechanisms, some examples of tunable lasers that have been commercialized, and a discussion of control techniques. More extensive data is given for the widely-tunable sampled-grating distributed-Bragg-reflector (SGDBR) type of laser, including data for such lasers integrated monolithically with modulators to form complete transmitter front ends. A summary of reliability data for the SGDBR laser is also given. It is concluded that tunable lasers can reduce operational costs, that full-band tunability is desirable for many applications, that monolithic integration offers the most potential for reducing size, weight, power and cost, and that sufficient reliability for system insertion has been demonstrated.

Reduced optical scattering loss in vertical-cavity lasers using a thin (300 /spl Aring/) oxide aperture

Calculations show that significant optical scattering loss persists as standard quarter-wave (800 /spl Aring/) thick, dielectrically apertured vertical cavity laser diameters are reduced below 4 /spl mu/m and that thinner apertures can reduce the scattering loss, By using a thin (300 /spl Aring/) AlAs-oxide defined aperture, optical scattering loss has been dramatically reduced over the quarter-wave AlAs-oxide defined vertical-cavity laser. The optical loss reduction results in 2.3 /spl mu/m diameter lasers with differential efficiencies of 0.43 (80% of the value of broad-area lasers) and continuous-wave single-mode powers of 1.2 mW.

Postgrowth tuning of semiconductor vertical cavities for multiple-wavelength laser arrays

Combined lateral-vertical oxidation of AlGaAs is investigated as a means of tuning the resonant wavelength of a semiconductor microcavity after the epitaxial growth. It is shown that this technique can provide arrays with a wavelength spread equal to the cavitys free spectral range with a single postgrowth processing step. Design issues for multiple-wavelength vertical-cavity laser arrays using this postgrowth tuning technique are discussed, comparing the performance of devices with all-semiconductor and partially or totally oxidized Bragg mirrors. Experimental results are presented on arrays with a 48-nm lasing span around 970 nm, using partially and totally oxidized mirrors.

Low-temperature optimized vertical-cavity lasers with submilliamp threshold currents over the 77-370 K temperature range

We demonstrate an extended temperature range (77-370 K) of continuous wave (CW) operation for dielectrically-apertured double-intracavity-contacted vertical-cavity InGaAs strained QW lasers optimized for operation at cryogenic temperatures. Superior performance is achieved through the alignment of the cavity mode with the gain of the first and second quantized subbands at 77 K and room temperature, respectively. This design results in submilliamp threshold currents over a 77-370 K temperature range for 5.4-/spl mu/m diameter lasers. The threshold is 120 /spl mu/A and the output power is >8 mW at 77 K.

Recent advances and important issues in vertical-cavity lasers

The rapid pace of advances in vertical-cavity surface- emitting lasers (VCSELs) has continued over the past couple of years. The widespread use of dielectric apertures formed primarily by lateral oxidation has provided much lower cavity losses, and this has enables a large decrease in device threshold as well as an increase in efficiency. The lowest optical losses have been obtained with thin or tapered oxide apertures. Within the past year, new strained- layer materials such as AlGaInAs have been incorporated to extend the benefits of strain to the 850 nm wavelength range. A record threshold of 290 (mu) A at 840 nm has been obtained. Devices have been designed for ultra-wide operating temperature ranges by using gain from different quantum levels at different temperatures. Submilliamp thresholds from 77 K to 373 K were demonstrated. The inclusion of low-loss dielectric apertures in wafer-bonded 1.55 micrometer InP/GaAs has yielded VCSELs with submilliamp thresholds for the first time. In addition, there has been considerable effort in making VCSEL arrays for parallel or free-space interconnect applications. Multiple wavelength arrays for even denser interconnects or wavelength addressing schemes have also been explored. In this paper we review some of this recent progress and point out issues still inhibiting further advances.

Multiple-wavelength vertical-cavity laser arrays based on postgrowth lateral-vertical oxidation of AlGaAs

We demonstrate that combined lateral-vertical oxidation of AlGaAs can be used to change the resonant wavelength of an optical cavity after the single epitaxial growth. A multiple-wavelength array of vertical-cavity surface-emitting lasers with a 48 nm wavelength span has been realized using this technique.

VCSELs in '98: what we have and what we can expect

Recent results from the authors group are summarized as a general indicator of the current state of the art in vertical-cavity surface-emitting lasers (VCSELs). These include results from engineered-aperture VCSELs with high wall-plug efficiencies at low powers, high-efficiency bottom-emitting cryo-VCSELs with wavelengths < 900 nm, low-threshold AlGaInAs VCSELs emitting at 850 nm, and arrays of VCSELs used in parallel free-space links as well as WDM arrays butt-coupled to multimode fiber. Analysis indicates that size-dependent losses limit the scaling of VCSELs below 5 micrometers in diameter unless special engineered apertures and/or short cavities are used. The analysis also shows that lateral carrier confinement is necessary to obtain efficient devices below 2 micrometers in diameter.

Electrical and optical losses in dielectrically apertured vertical cavity lasers

In this paper, we measure the size dependent optical scattering and electrical losses in etched-post and dielectrically apertured vertical-cavity lasers (VCLs). We show that reduced optical scattering losses are responsible for the dramatic improvement in device scaling seen with the use of the oxide-defined apertures. Furthermore, we experimentally show how to reduce this optical scattering loss through the use of thin apertures. We find that the electrical losses (due to current leakage around the active region and carrier diffusion in the active region) in the structures are minimized by reducing the doping near the active region, minimizing the current leakage. Finally, based on the experimental results, theoretical design curves for VCL scaling are calculated.

Substrate-emitting short-wavelength vertical-cavity lasers for cryogenic optical interconnects

We report on a substrate emitting 885-nm (77 K) vertical cavity lasers (VCL) optimized for operation at cryogenic temperature. Submilliampere threshold currents over a 77-300 K temperature range for 3.8-/spl mu/m-diameter devices and power conversion efficiencies >30% for larger devices at cryogenic temperatures are demonstrated. The investigated VCL structure is shown. The active region contains two 60-/spl Aring/ InGaAs-GaAs quantum wells (QWs).

Postgrowth tuning of cavity resonance for multiple-wavelength laser and detector arrays

Multiple-wavelength arrays of vertical-cavity lasers (VCLs) and detectors are important for wavelength-division-multiplexing systems. In-plane variation of the cavity resonant wavelength usually requires growth on patterned substrates or two successive growths and a complicated processing. It was recently observed that the lateral wet oxidation of an AlGaAs layer can result in partial vertical oxidation of an adjacent layer. We propose to use this combined lateral-vertical oxidation to change the optical thickness of a cavity after the epitaxial growth of both Bragg mirrors.