Page 176 - Tunable Lasers Handbook

4 CO, Isotope Losers and Their Applications 155
Electro-optic waveguide modulators for frequency tuning of CO, - (and
other infrared) lasers provide a second method of obtaining a continuously tun-
able cw signal source between adjacent CO, lasing transitions. The develop-
ment of such modulators was pioneered by-Cheo, who in 1984 reported as
much as a 30-GHz total frequency tuning range in two sidebands from a line-
selectable CO, laser by phase modulation of an optical guided wave in a thin
GaAs slab active layer at microwave frequencies [132-1351. More recent
advances in electro-optic waveguide modulators for generating tunable side-
band power from infrared lasers was also published by Cheo in 1994 [136].
Some of the high-resolution spectroscopic measurements obtained with these
modulators are described in [137,138].
The third type of continuously tunable cw signal source is provided by a
family of lead-salt tunable diode lasers (TDLs). Undoubtedly. these lasers are by
far the most versatile and widely used sources of tunable IR radiation: however.
their power output is rather limited, usually below a few milliwatts. Also. their
use requires cryogenic cooling, and achieving tunable single-frequency output is
often a problem. On the other hand, even a single TDL can provide an enormous
tuning range.
The first lead-salt TDLs were made at MIT Lincoln Laboratory by Butler er
al. in 1964 [139.140]. An excellent short review of the MIT Lincoln Laboratory
work on TDLs was written by Melngailis in 1990 [141].
The early MIT Lincoln Laboratory work included the first optical heterodyne
detection of beat frequencies between a tunable Pbo.88Sno,,,Te diode laser and a
(second-generation) ultrastable CO, laser by Hinkley er nl. in 1968 11321. Shortly
thereafter the first direct observation and experimental verification of the quantum-
phase-noise-limited linewidth predicted by Schawlow and Townes in 1958 [57]
was demonstrated by Hinkley and Freed also using a Pbo.ssSno~,,Te TDL hete-
rodyned with the same CO, laser as described earlier [143]. This fundamental
quantum-phase-noise-limited Schawlov+Townes linewidth was subsequently reaf-
firmed from spectral analysis of the beat frequencies between a solitary PbSl--xSe~y
TDL and an ultrastable (third-generation) CO laser by Freed et al. at MIT Lincoln
Laboratory in 1983 [ 1441. Linewidths as narrow as -54 kHz at 10.5 pm [ 1431 and
-22 kHz at 5.3 pm [ 1441 were achieved with the above-mentioned lead-salt TDLs.
Figure 23 illustrates the emission wavelength (wave number) range of lead-salt
TDLs and some of the compounds used to fabricate such devices.
The reasonably narrow linewidths, the ability to produce devices at any
required wavelength to match molecular absorption lines, and the capability of
short-range tuning through variation of the injection current opened up semiconduc-
tor laser applications in high-resolution spectroscopy and air pollution monitoring.
These applications provided the impetus for the creation in 1974 of the first spin-off
from Lincoln Laboratohy in the laser area, Laser Analytics (presently lmown as ha-
lytics Division of Laser Photonics. Inc.). To the best of my knowledge this c~mpany
is the sole US. manufacturer of lead-salt TDLs, since MIT Lincoln Laboratory