Grating-out-coupled Surface-Emitting (GSE) Lasers

 

The basic concept of a datacom/telecom GSE laser is shown in cross-section in Fig. 1.  These GSE lasers use a planar cavity with first-order DBR gratings on both ends for feedback and a second-order grating in the middle (Fig. 1a) or near one end (Fig. 1b) of the device to out-couple light.  Because the lasing wavelength is selected by a first order grating in a passive region that has the same effective index and temperature dependence as the outcoupling region, the outcoupled beam is stable over temperature and current.  (This would not be the case if both DBRs were replaced by broadly reflecting cleaved facets.)

        One of the shallow DBRs could be replaced with an extremely deep DBR (as shown in Fig. 1b) that would give a very high and broad reflectivity in less than ten microns [8].  The emitting aperture is ~ 10 µm to 15 µm longitudinally and ~ 6 µm laterally for efficient out-coupling into single mode fibers.

(a)

(b)

Fig. 1. a) Basic concept of a GSE laser with an out-coupling grating within the gain region. b) GSE laser with an out-coupling grating at the end of the gain region and one deep and one shallow grating DBR on each side.

The SEM micrograph in Fig. 2a shows the end of the ridge-guide on the left, the grating-out-coupler in the center, and the beginning of the DBR on the right for a 1310 nm GSE laser.  A close-up of the out-coupler grating (period is ~ 400 nm) is shown in Fig. 2b.

L-I curves measured by butt coupling a GSE laser to a multi-mode fiber over a range of temperatures and to single-mode fiber are shown in Fig. 3a,b.  The spectrum (Fig. 3c) is single-frequency with > 40 dB SMSR.  The device has a 380 µm long gain section, 15 µm long outcoupler and 200 µm long DBRs.  The threshold is less than 20 mA (best on-wafer thresholds are 13 mA), and the multi-mode fiber coupled slope efficiency is ~ 0.06 mW/mA (best values are 0.1 mW/mA).  The efficiencies by butt coupling (no lenses) are ~ 100% to multimode fiber and ~ 45% (best value is 52%) to single-mode fiber.  The wavelength variation and side-mode suppression-ratio (SMSR) as a function of current for various temperatures are shown in Fig. 3d,e.

(a)

(b)

Fig. 2. SEM micrographs of an out-coupling grating. a) top view, b) side view.

(a)	(b)		(c)
d)		e)

Fig. 3.  Wafer level a) light-current curves; b) fiber-coupled light-current curves; c) spectra for a 1318 nm GSE laser; d) wavelength as a function of current at various temperatures; and e) SMSR as a function of current over various temperatures.

Laser operation can continue beyond 85C, with only a small reduction in the slope efficiency. The "snap-on" behavior at threshold, which shows up with increasing temperature is due to a combination of low DBR reflectivity (~40%/DBR) and the saturable absorption of the quantum wells in the passive DBR and outcoupler regions.Shifting the peak reflectivity of the DBRs by +8 nm raised the snap-on onset from 45C to 65C with a ~ 1 mA increase in threshold at 25C. Along with increasing the DBR reflectivity from about 40% to > 80%, shifting the DBR peak reflectivity will eliminate the snap-on characteristic below 85C. Disordering the quantum wells in the passive outcoupler and DBR sections will further reduce the internal losses, lower threshold currents and allow operation beyond 100C. Reliability testing in excess of 1000 hours at 85C at a 100 mA bias current of 1310 nm GSE lasers has shown little degradation.

 

The far-field (Fig. 4) beam divergence of the device is 3.5 x 8 degree (FWHP) and 8 x 20 degree (1/e²). Figure 5 shows an open eye diagram for this 1320 nm GSE laser modulated with a 2³¹-1 pseudo-random bit sequence (PRBS) signal at 3.125 Gbps. The bit error rate (BER) measured with a back-to-back fiber link is below 10^-11.  Future packaging efforts and device development are directed at 4 and 10 Gbps modulation rates.

 

Fig. 4. 2D view of Far-field scan of a 1310 nm GSE laser with a 5 um wide ridge and a 15 um long OC grating.	Fig. 5. 3.125 Gbps eye diagram of a 1318 nm GSE laser.
(a)	(b)

 Fig. 6. a) GSE laser with a top reflector and an AR coating on the substrate and b) an AR coated integrated lens.

The present GSE emits from the top, wasting ~ half of the power. Fabricating a metallic reflector on the top and providing an anti-reflective (AR) opening in the substrate, directs the generated light through the substrate (Fig. 6a).  Additionally, a lens can be integrated on the substrate, eliminating the lens in the package (Fig. 6b). A simple Monte Carlo packaging analysis of a GSE with an integrated monolithic lens indicates that a passive packaging process can couple ~80% of the light into a single mode fiber with a yield of ~90 %.