Page 458 - Tunable Lasers Handbook

41 8 Paul Zorabedian

maximization of the external feedback strength and axial mode selectivity along
with minimization of the residual facet reflectance help to reduce multimoding.
Limiting the drive current to within twice the threshold current also lessens the
problem. These measures may not be wholly effective. owing to the properties of
the cavity components and the facet coating, or to the required output power. In
that case, it may be necessary to implement one of several reported methods for
multimode detection and suppression. The methods can be divided into two cate-
gories, optical and electrical, according to whether the control signal is applied
to the external cavity or the diode laser.

13.1 Optical Multimode Suppression Methods

Two elements are needed to implement an automatic control loop for niulti-
mode suppression: (1) a means for deriving an error signal and (2) an effective
control variable.
The presence of multiple longitudinal modes in the output spectrum can be
directly observed with a scanning Fabry-Perot interferometer. With this instru-
ment. however, it is difficult to detect sidemode ratios less than --20 dB. In
addition, some kind of software or firmware interpretation of the spectrum,
either with a computer or a digital oscilloscope, is necessary to use this informa-
tion as the error signal to drive a control loop. Alternatively, the microwave
mode-beating signal from a wide-band detector is a much more sensitive indica-
tor of multimoding and its output can be more easily used in a control loop. Het-
erodyne amplification gives this detection method a sidemode-ratio sensitivity
--60 dB or better.
As for control variables, small changes in the cavity length have been
observed to shift the laser's output spectrum from multimode to single mode. A
possible explanation for this phenomenon is that when one mode is shifted to
coincidence with the wavelength of minimum external-cavity loss, the differen-
tial loss for competing sidemodes is maximized. and they are suppressed.
Another external-cavity control mechanism requires the use of an intracavity
etalon to supplement the wavelength selectivity of a diffraction grating. Control-
ling the angle of the etalon so that its loss minimum corresponds with that of the
grating is also effective for sidemode suppression.
A multimode suppression servo system for a Littrow-grating ECL was
reported [ 1201 that used the rf beat signal generated by multiple longitudinal
modes as an error signal. The error signal was fed into an integrator whose out-
put was used to drive the axial position of the grating using a PZT (Fig. 41). The
published implementation used a Mach-Zender interferometer with unequal path
lengths to create temporal overlap between modes. I have built similar systems
using the mode-beating error signal to drive either the cavity length or the angle
of an intracavity etalon and have found that sufficient error signal could be gen-
erated without the differential-delay interferometer.