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8 Tunable External-Cavity Semiconductor Lasers 399

8. I .3 Grating-Tuned Ring External Cavities
The first report of a grating-tuned ring ECL operating in strong-feedback
mode was by Bogatov and coworkers [86]. The active element was an AlGaAs
heterojunction optical gain medium with a stripe tilt angle of 15". The residual
facet refiectance was estimated at no greater than 0.01%. The cwity comprised
two 0.5 NA. 15-mm focal length collimating objectives. two mirrors, a 600 l/mm
grating, and an intracavity etalon. With the grating only, the output spectrum
consisted of about 20 external-cavity modes. With the insertion of the etalon, the
laser operated in a single axial mode. Round-trip cavity loss was not given. and
no tuning range was reported.
Oshiba and coworkers described a 1300-nm semiconductor fiber ring laser
tuned with a bulk optic grating [87]. The laser contained an optical isolator bo
force unidirectional traveling-wave oscillation. Coupling between the 500-ym-
long amplifier and the polarization-maintaining fiber was done with ball lenses.
The input and output beams of the grating were coupled to the fiber using GRIN

TABLE 6 Grating-Tuned Extended-Cavity Lasers

Wavelength Configuration Comments Reference

850 nrn Littrow One of the first papers demonstrating essential [81] (1972)
features of strong. \Va\,eleneth-selecti\.e feedback;
cryogenic cw GaAs homojunction laser diode:
h!4 Si0 AR coating,f/2.5 lens: 15-nm tuning rarge
1.55 pm Littrow Intracavity tilting plate for fine tuning, [82] (1985)
"shoebox-size" package
850 nm Littroa 32-nm tuning range [I61 (1985)
1300 nm Littrou. Short cavity: GRIN rod lens. prism grating [83] (19871
1.55 pm Littroa Lensed fiber output coupling. piezoelectric [81] (19881
cavity length control for fine tuning; "palm-size"
package served as the prototype for a product
marketed by BT&D (now Fiber Optic Components
Operation of Hewlett-Packard)
1.3 pni Littrow Use of intracavity cylinder lens to illuminate [60] (19901
grating with narrow stripe beam for improved
tolerance to angular misalignment (one-
dimensional quasi-degenerate resonator) (Fig. 27)
780 nm Grazing incidence Grazing-incidence cavity using zeroth-order [85] (19911
grating reflection for output coupling (Fig. 28)
1.3 prn Littrow GRIN rod lens collimator, intracavit) silicon [61] (1992)
prism beam expanders (Fig. 29)
850 nm Littrow Tapered-waveguide gain chip. 1-W air beam [13] (1993)
output (Fig. 30)