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Lasers & Sources

Narrowing the linewidth of 852nm diode lasers

Narrow-linewidth tunable semiconductor lasers are needed for communication and spectroscopy applications. AlGaAs/GaAs 852nm lasers were recently demonstrated with narrow linewidths provided by first-order distributed Bragg reflectors.
25 May 2006, SPIE Newsroom. DOI: 10.1117/2.1200603.0177

Tunable semiconductor lasers with narrow spectral linewidths at specific wavelengths are of interest for both communication and spectroscopy applications. In communications, narrow-linewidth lasers are necessary to reduce optical dispersion, enabling high-speed data rates. Such lasers can also be used for spectroscopic applications that require tuning to the specific wavelengths of an atomic transition. The optical pumping of a cesium transition at 852nm is of particular interest for applications like optical inertial guidance systems. Both types of applications require lasers that operate with a single optical mode and that exhibit spectral linewidths narrower than 1MHz. Most edge-emitting semiconductor lasers, however, have multiple longitudinal modes spaced closely together. Incorporating a distributed Bragg reflector (DBR) or distributed feedback (DFB) grating selects a single longitudinal mode, which allows the laser to operate in a single spectral mode.

DBR and DFB gratings are typically located at the interface between the core and cladding of a laser to provide the necessary feedback for narrow-linewidth performance. However, locating the grating between the core and cladding requires epitaxial regrowth, which is difficult for devices with AlGaAs barriers due to the rapid oxidation of Al-containing compounds. Surface-grating ridge-waveguide DBR lasers have been developed as a method for achieving narrow-linewidth lasers with a single epitaxial-growth step.1 By incorporating an asymmetric cladding, one can reduce the etch depth and form first-order gratings in the DBR section.2,3 These devices exhibit narrow-linewidth operation with a minimum spectral linewidth of 36kHz, as determined by the self-heterodyning measurement technique. We recently reported4 fabricating narrow-linewidth 852nm asymmetric-cladding ridge-waveguide DBRs with first-order gratings in the AlGaAs/GaAs material system.

We used metallorganic chemical vapor deposition to grow an asymmetric-cladding separate-confinement heterostructure. The growth consisted of a more-or-less-conventional heterostructure design but with a thinner-than-usual (400nm) p-cladding layer. We grew samples with gain peaks centered at 835nm and 850nm. We patterned DBR gratings using electron-beam lithography and transferred the gratings to SiO2 via reactive-ion etching. An inductively-coupled-plasma reactive-ion etch transferred the pattern to the upper cladding of the laser structure. Figure 1(a) shows a scanning electron micrograph of the resulting 350nm etch. Then we formed the ridge waveguide, as shown in Figure 1(b). The 400nm-thick upper cladding was chosen to increase the effective-refractive-index step, which reduced the etch depth required for strong coupling. We chose the metals to reduce the optical loss due to modal overlap with the metal adhesion layers. We formed contacts with the laser-gain, DBR-tuning, and phase-tuning sections. The complete structure is shown in Figure 1.

Figure 1. Scanning electron micrographs of the (a) top view and (b) cross section of the first-order distributed Bragg reflector (DBR) grating. The schematic diagram shows structure of the asymmetric-cladding DBR laser diode.

The peak continuous-wave (CW) output powers emitted from the cleaved facet end of these devices exceeds 25mW. The threshold current is 22mA for unbiased DBR and phase-tuning sections. Figure 2 shows the mode spectrum of the both devices operating CW with an injected current of 40mA into the gain section. The peak emission wavelength of this device is centered at 854nm with a side-mode-suppression ratio (SMSR) of more than 35dB. The devices with a peak gain at 835nm had a threshold, but both devices lased at essentially the same wavelength, an indication of the strong coupling of the surface-etched DBR grating.

Figure 2. Output spectrum and lasing modes for devices with peak gain at 835nm and 850nm.

The spectral linewidths of the ridge-waveguide DBR lasers were measured by a delayed self-heterodyning method, as first proposed by Okoshi et al.5 Spectral lineshape measurements were taken at various output powers, with minimum measured linewidths of between 30 and 90kHz for each device. For output powers between 5 and 25mW, the spectral linewidth remains below 200 kHz. At higher power levels the laser outputs rebroaden to values in the 1MHz range. This is predicted theoretically6,7 and has been observed with similar devices fabricated at 1005nm.3

The laser cavity is strongly coupled to the DBR by the large index step and strong metal interaction. This strong DBR coupling increases the effective cavity lifetime, increasing the quality factor (Q) of the cavity. This suggests that the narrow spectral linewidths are a result of the cavity (as opposed to improvements in the linewidth-enhancement factor).

These results demonstrate that lasers with narrow spectral linewidths for spectroscopy can be obtained using surface-etched DBR gratings in the GaAs/AlGaAs material system.

The authors would like to acknowledge support by the Defense Advanced Research Projects Agency (DARPA) DSO PINS Program, the National Science Foundation (ECS-0335082), and the Intel Alumni Endowed Chair in Electrical and Computer Engineering.

James J. Coleman, R.K. Price, and V.C. Elarde   
Department of Electrical and Computer Engineering, University of Illinois
Urbana, IL
James J. Coleman holds the Intel Alumni Endowed Chair in Electrical and Computer Engineering at the University of Illinois in Urbana. In addition, he has presented ten invited papers at SPIE conferences over the years and has often served on program committees for Photonics West and Optics East.