Edge-emitting superluminescent diodes (SLDs)

Edge-emitting LEDs are motivated by the need for high-brightness LEDs that allow for high- efficiency coupling to optical fibers. Edge-emitting LEDs comprise an optical waveguide region that guides light emitted along the waveguide by total internal reflection.

Superluminescent light-emitting diodes or superluminescent diodes (SLDs) are broad-band high-intensity emission sources that emit incoherent light. Incoherent light does not result in “speckle patterns” obtained from coherent light sources such as lasers. SLDs are suitable as communication devices used with single-mode fibers and also as high-intensity light sources for the analysis of optical components (Liu, 2000).

Light is guided in the core region of the waveguide. Total internal reflection occurs at the boundaries between the core region and the upper and lower cladding layers as shown in Fig. 23.8. In order to make waveguiding possible, the core layer must have a higher refractive index than the cladding layers. Photons emitted with a sufficiently small angle of incidence at the core-cladding interface will be guided by the waveguide, as indicated in Fig. 23.8.

Since the light is guided by the waveguide, the light intensity emitted by the device linearly increases with the length of the waveguide. Thus, increasing the length of an edge-emitting LED allows one to obtain a higher light output intensity. However, the electrical current required to drive the LED also increases with the stripe length.



— Recombination events

Upper confine­ment layer •

Active layer -

Lower confine-' ment layer

Substrate -

Upper cladding layer

— Waveguide core layer Lower cladding layer

— Substrate

Fig. 23.8. Waveguide geometry showing guided light rays in the core layer with low angles of incidence.

Superluminescent diodes are edge-emitting LEDs that are pumped at such high current levels that stimulated emission occurs. In the stimulated emission process, one photon stimulates the recombination of an electron-hole pair and the emission of another photon. The photon created by the stimulated emission process has the same propagation direction, phase, and wavelength as the stimulating photon. Thus SLDs have greater coherence compared with LEDs. In the stimulated emission regime, spontaneous emission towards the top surface of the LED is reduced and emission into waveguide modes is enhanced.

SLDs are quite similar to semiconductor laser structures with one important difference: SLDs lack the optical feedback provided by the reflectors of a semiconductor laser. Two typical SLD structures are shown in Fig. 23.9. The SLD structure shown in Fig. 23.9 (a) has a reflective back­side reflector facet; the front-side facet, however, is coated with an anti-reflection (AR) coating. To prevent lasing, the front-side facet must have a reflectivity of < 10-6 (Liu, 2000; Saul et al., 1985). Exceeding the required reflectivity results in unwanted lasing of the device. Owing to the high-quality AR coating requirement, SLDs with an AR coating are expensive to manufacture.

A lower-cost alternative SLD structure is shown in Fig. 23.9 (b). This structure uses a lossy region near the back-side facet of the diode. The lossy region is not covered by the top metal contact and thus is not pumped by the injection current. Practically no feedback occurs if the length of the lossy region is much longer than the absorption length of the core region, i. e.

length of lossy region >> a-1 , (23.1)

where a is the absorption coefficient of the core region. The absorption coefficient in III-V semiconductors near the band edge is a « 104 cm-1. Thus for lossy-region lengths exceeding several tens of micrometers, the optical feedback from the back-side facet is negligibly small.

In addition to absorption losses, diffraction losses occur in the region not pumped by the electrical current. Gain guiding occurs in the region injected by the electrical current but not in the lossy region. Thus both absorption and diffraction losses prevent this type of SLD from lasing.

(a) LED (b) Laser (c) AR-coated SLD (d) Ideal SLD

Wavelength X Wavelength X Wavelength X Wavelength X

Fig. 23.10. Spectrum of (a) regular LED with a theoretical linewidth of .SkT, (b) multi­mode semiconductor laser, (c) superluminescent diode (SLD) fabricated by AR coating of a multimode laser, (d) ideal SLD with linewidth less than kT (after Liu, 2000).

Emission spectra of an LED, SLDs and a laser are shown in Fig. 23.10. The LED has a broad spontaneous emission spectrum. The spectrum of an SLD with a residual small facet reflectivity exhibits periodic oscillations in the emission spectrum due the Fabry-Perot cavity enhancement. An ideal SLD has a smooth spectrum and does not exhibit any oscillations. The spectral width of SLDs is narrower than that of LEDs due to increased coherence caused by stimulated emission. Also shown is the spectrum of a Fabry-Perot laser with several laser modes.

Current Current Current

Fig. 23.11. Light-versus-current (L-Г) characteristic of different LEDs, (a) Edge-emitting LED with little or no saturation effects, (b) Surface-emitting LED with small active area exhibiting saturation effects due to carrier overflow, (c) Superluminescent LED. Also shown is the L-I characteristic of a laser that exhibits a distinct threshold current.

A comparison of the L-I curves of an LED, an SLD, and a laser is shown in Fig. 23.11. Surface-emitting LEDs with a small light-emitting region diameter, tend to have sublinear L-I characteristics. At high injection current densities, the small active volume of surface-emitting LEDs is swamped with carriers leading to saturation. Edge-emitting LEDs operating in the spontaneous emission regime have linear L-I characteristics, as expected for ideal LEDs. SLDs have a superlinear L-I characteristic due to stimulated emission. In the stimulated emission regime, an increasing number of photons are guided by the waveguide. The number of photons emitted into waveguide modes increases with injection current as stimulated emission becomes dominant. As for SLDs, semiconductor lasers have superlinear emission characteristics. However, the L-I curve of lasers exhibits a more distinct threshold than that of SLDs.

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