8 Tunable External-Cavity Semiconductor Lasers
~ Filter Nd:YV04 KTP Output mirror
nrn /,,-,~ ~,,
532 nm
I Collimating Focusing lens 1064 nm
Laser diode lens
Interference-filter-tuned extended-cavity laser used to pump intracavity-doubled
Nd:YVO 4. (Reproduced with permission from Kitaoka et al. [163].)
18.7 Injection Seeding
The optical parametric oscillator (OPO) is arguably the most widely tunable
coherent optical source. However, it is difficult to obtain narrow-bandwidth out-
put from an OPO. Use of dispersive elements in the OPO cavity complicates
tuning. An alternative is to use a tunable ECL as an injection-seeding source. A
1.55-~tm grating ECL with a 150-kHz linewidth has been used to seed a lithium
niobate OPO [164]. The OPO was pumped at 1.064 ~tm with a Nd:YAG laser.
Seeding reduced the bandwidth of the signal from 50 GHz without seeding to
0.18 GHz with seeding. Seeding was obtained for injected signal wavelengths
from 1.526 to 1.578 lam, corresponding to an idler wavelength range of 3.26 to
3.51 ~tm. The authors noted that these limits could be extended with improved
cavity optics and a different seeding source. The tunable idler radiation is useful
for atmospheric spectroscopy because many species absorb in the 3- to 4-~tm
atmospheric transmission window.
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Linewidth in DFB Lasers with Superstructure Grating (SSG)," Electron. Lett. 29, 1350-1351
(July 1993).
2. Y.C. Chung and Y.H. Lee, "Spectral Characteristics of Vertical-Cavity Surface-Emitting
Lasers with External Optical Feedback," Photon. Technol. Lett. 3, 597-599 (July 1991).
3. S. Jiang, Z. Pan, M. Dagenais, R. A. Morgan, and K. Kojima, "Influence of External Optical
Feedback on Threshold and Spectral Characteristics of Vertical-Cavity Surface-Emitting
Lasers," Photon. Technol. Lett. 6, 34-36 (Jan. 1994).
4. The internal quantum efficiency
is defined as the ratio of the total carrier lifetime x c to the
radiative lifetime 't r.
5. C.H. Henry, "Theory of the Linewidth of Semiconductor Lasers," IEEE J. Quantum Electron.
QE-18, 259-264 (1982).
6. L. D. Westbrook, "Dispersion of Linewidth-Broadening Factor in 1.5 ~tm Laser Diodes," Elec-
tron. Lett. 21, 1018-1019 (1985).
7. K. Naganuma and H. Yasaka, "Group Delay and m-Parameter Measurement of 1.3 lam Semi-
conductor Traveling-Wave Optical Amplifier Using the Interferometric Method," IEEE J.
Quantum Electron. 27, 1280-1287 ( 1991 ).

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