Ridge-waveguide laser
An edge-emitting semiconductor laser whose lateral waveguide is a ridge, typically 2–5 µm wide, etched into the upper cladding above the active layer. The ridge confines the current and creates a small lateral effective-index step, a few times 10⁻³, that guides a single lateral mode.
A ridge-waveguide laser is an edge-emitting laser in which a narrow ridge, typically 2–5 µm wide and a few hundred micrometers to a few millimeters long, is etched into the upper cladding of the epitaxial structure. The metal contact sits on top of the ridge, so current enters the active layer only below it, and the extra cladding thickness under the ridge raises the local effective index, forming a weak lateral waveguide with an index step of a few times 10⁻³. Most single-mode edge-emitting lasers, including 980 nm pump lasers and many telecom Fabry-Perot and DFB lasers, use this structure because it needs one epitaxial growth and a single etch.
Index guiding and gain guiding
The earliest stripe lasers were gain guided: a narrow contact stripe defined where current flowed, and the lateral mode was held only by the gain profile. Carriers lower the refractive index, by about −1.5 × 10⁻²⁰ cm³ per carrier, so the pumped region has a lower index than its surroundings and acts as a weak antiguide. Gain-guided lasers have curved lateral wavefronts, a lateral beam waist that lies behind the facet and so a strongly astigmatic output, and a lateral mode that changes shape with current.
A ridge adds a built-in index step that does not depend on the carriers. Above threshold the average carrier density is clamped, but its lateral profile can still change; a variation of 10¹⁷ cm⁻³ corresponds to an index change of −1.5 × 10⁻³, comparable to the built-in step of a shallow ridge; this is why such ridges are called weakly index guided, and why their behavior at high current depends on carriers and heat as well as on the etch.
Etch depth and the index step
The lateral index step is usually estimated with the effective index method: the vertical slab under the ridge and the thinner slab beside it each give an effective index, and their difference is the step of an equivalent lateral slab waveguide. Stopping the etch farther above the active layer leaves more cladding beside the ridge and a smaller step; etching closer gives a larger one. For shallow ridges the step changes steeply with the residual cladding thickness, so the etch depth is tightly controlled, often by an etch-stop layer. Etching through the active layer produces a deep ridge with a large index step and no lateral carrier diffusion beyond the ridge, at the cost of surface recombination at the exposed active layer, which matters more in GaAs than in InP. Passive silicon guides make the same shallow-versus-deep distinction under other names (strip and rib waveguides).
Ridge width and single-mode operation
Treating the lateral guide as a symmetric slab with , only the fundamental lateral mode is guided when the ridge width satisfies
At 980 nm with , this cutoff width is 3.5 µm for , 2.7 µm for , and 1.9 µm for . Ridges are often made at or above the cutoff, because a narrower ridge raises the series resistance, the optical intensity at the facet and the lateral far-field angle. The first-order lateral mode then exists but has more loss and less overlap with the gain than the fundamental, and stays below threshold only up to a certain current. When it reaches threshold, or locks in phase with the fundamental, the beam steers sideways by roughly a degree and the light-current curve shows a kink; the mechanism and its measurement are covered in Why L-I curves kink.
The lateral near field of a single-mode ridge is a few micrometers wide. A Gaussian mode of 1.5 µm 1/e² half-width at 980 nm diverges with a 1/e² half-angle of 11.9°, a full width at half maximum of 14° in the slow axis; the vertical (fast-axis) divergence is larger, which gives the familiar elliptical far field.
Characterization
The ridge width and etch depth are measured by cross-section scanning electron microscopy or by profilometry before metallization. On a finished device, near-field and far-field scans in the lateral direction show whether a single lateral mode is present at each current, and the derivative of the light-current curve locates kinks. Fiber-coupled light-current curves reveal lateral-mode changes that an integrating sphere hides.
Common questions
How does a ridge laser differ from a buried-heterostructure laser?
In a buried heterostructure the active layer is etched into a narrow stripe and overgrown with higher-bandgap material, which gives strong lateral index guiding and confines the carriers as well as the light. A shallow ridge leaves the active layer continuous, so carriers can diffuse sideways beyond the ridge edges (carrier confinement), and the threshold is higher; in exchange it needs no regrowth.
What ridge width gives a single-mode laser?
For a weakly guided ridge at 980 nm, roughly 2–4 µm, depending on the index step set by the etch depth. Wider stripes, tens to hundreds of micrometers, are broad-area lasers that run in many lateral modes at higher power.
Why does the output of a ridge laser shift sideways at high current?
Spatial hole burning and heating change the lateral index profile until the first-order lateral mode lases with the fundamental; the two modes beat, and the near field shifts and the far field steers by about a degree while the total power changes little.
References: L. A. Coldren, S. W. Corzine and M. L. Mašanović, Diode Lasers and Photonic Integrated Circuits, 2nd ed. (Wiley, 2012). G. P. Agrawal and N. K. Dutta, Semiconductor Lasers, 2nd ed. (Van Nostrand Reinhold, 1993). M. F. C. Schemmann et al., "Kink power in weakly index guided semiconductor lasers," Applied Physics Letters 66, 920 (1995).