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Mode Stabilization Technique for the Multifrequency
L. Mtiller, C.R. Doerr, C.
Bell Labs, Lucent Technologies, 791 Keyport-Holm
ner, M. Zirngibl
07733 New Jersey,
Multifrequency lasers (MFL) are attractive sources for wavelength division multiplexed (WDM)
systems because they have a very large tuning range and can be used for simultaneous multiwavelength operation [1,2]. The whole transmission window of an EDFA can be cwered by one laser
chip containing around 16 channels by combining geometrical tuning with temperature tuning.
However, inherent disadvantage of the MFL is that it requires a lot of real estate on InP wafers. Also,
in order to guarantee single mode stability, a narrow filter width is required [3]. In this paper we
demonstrate a novel method for stabilizing the modal characteristics even for designs with large filter
bandwidths which, in turn, allows one to significantly reduce the size of the chip and increase the
direct modulation rate.
la. Laser modes depending on their
The operation principle of a convent
frequency are separated or combined by means of a waveguide grating router (WGR) which filters
light of different wavelengths to different output ports. Semiconductor Optical Amplifiers SONS
connected to these ports amplify the waves until laser oscillation starts. Highly reflecting coatings at
the ends of the cavities increase the resonator Q.
The design of MFL's requires several trade-offs. First, one would like to have the filter bandwidth of
the WGR as narrow as possible, so that only a few longitudinal modes are lying in the WGR
passbands. Although nonlinear effects inside the laser cavities can support single mode operation, a
large sized WGR for generating narrow passbands is required [3]. As in a conventional gratings the
FWHM of a diffraction order is smaller the more grating lines are used per unit area. The~sizeand
open space used by a conventional MFL design on a chip is shown in the waveguide mask file graphic
in Fig. 1b. On the other hand the geometrical size of the laser should be as small as possible because
in a first order approximation the laser cavity length determines the maximum speed for data
modulation in aslongcavity laser, Typical cavity lengths are in the range of 1.5 cm corresponding to a
/s. Additionally small-sized laser chips are
more attractive du
a) Cartoon of a conventional ME.b) Scaled layout of a conventional MFL based on InP technology.
Novel technique for mode stabilization
With the implementation of an additional optical filter on the MFL chip we were able to demonstrate
single mode operation within wide bandwidth passbands of the WGR from a short cavity MFL placed
on a relatively smal1,chip. The filter technique is similar to that of the so-called DiDomenico-Seidel
cavity also know as the Vernier-Michelson Interferometer, which was originally invented for
longitudinal mode selection and stabilization in gas ers 141. It is based on an open-ended resonator,
shown in Fig. 2a consisting of two coupled cavitie
ich contain independently tunable amplifiers.
The resonator is a three arm device composed of three mirrors (MI, Mz, MO)and a beamsplitter (BS).
188 / TuL4-2
The two mirrors (MI, M2) at the cavity ends are 100% reflectors. The output mirror (MO) at the far
right can have any reflectivity less than 100%.The two modes in each cavity are combined in a 50%50% beamsplitter which allows the modes of the two subcavities to interact. The two subcavities are
defined by the MI, MO and M2, MO mirrors. The basic idea behind such an open ended cavity is that
only when the amplitudes, frequencies, and phases of both modes are matched they cancel each other
at the output port (OE) of the beamsplitter. Then the maximum amount of power remains in the laser
cavity at the design emission wavelength. In that resonant case all of the modal power is confined to a
narrow frequency range. The equilibrium between resonator losses and amplifier gain is then mainly
determined by the reflectivity of the output mirror and the saturation power of the SOA’s.
Fig. 2: a) DiDomenico-Seidel cavity used for longitudinal mode stabilization in gas lasers. b) Qualitative
depiction of the cavity losses due to radiation of the mode through port ‘OE’ of the beam splitter versus the
mode frequency.
Mismatching of the modal parameters would result in a drastic enhancement of the cavity losses. By
choosing slightly different cavity lengths L1 and L2, filter functions which look similar to those of
Fabry-Perot filters (FPF) can be realized (Fig. 2b). The free spectral range (FSR) determined by
/2(L1-L2), where c,ff stands for the phase propagation velocity, is chosen to be larger than the
gain bandwidth of the active media. Thus all modes except the main resonant mode lying in the filter
function peak and the gain bandwidth of the active media are suppressed.
To adapt this scheme to an MFL the WGR is used to perform two functions. First, using focal length
and parabolic chirp [5],the WGR limits the lasing frequency to one FSR of the DiDomenico-Seidel
resonator. Thus only one filter peak of the DiDomenico-Seidel cavity is lying in its net gain.
Secondly the WGR combines the two modes of each subcavity. WGR’s in conventional MFL’s are
designed to focus as much light as possible into one diffraction image to minimize the internal router
losses. Here we used an interleave chirped [6] WGR which generates two diffraction images of the
same intensity for each wavelength into two primary Brillouin zones. Both images are projected onto
output ports. Figure 3 shows the design steps of the DiDomenico-Seidel MFL concept. The passband
bandwidth of the FPF function transmission peak is much smaller than one of the WGR passbands
whereas the FSR of the FPF is comparable large to the WGR passband bandwidth. Hence, the
resulting total filter function is mainly dominated by the transmission peak of the FPF function.
A significant design difference to the classical DiDomenico-Seidel arrangement is given in our set up
by a third SOA located at the laser chip output. This SOA allows an additional phase and power
control of the laser mode and can be used as modulator.
Fip.3:Conventional MFL’s use one diffraction image of the WGR (a,b). Light of a second image is launched
into the second subcavity of the DiDomenico-Seidel resonator (b,c). Pairs of equal color points indicate arms
of one DiDomenico-Seidel cavity in the scaled layout of the chip mask (d).
Experimental results
For proof of principle a four channel DiDomenico-Seidel MFL based on an interleave chirped WGR
was designed. The WGR passband widths were -160 GHz which is 8 to 12 times wider than what a
TuL4-3 / 189
regular MFL requires for single mode operation. For
urement of the passband bandwidth the
SOA section was cleaved from the chip and thus pol
spontaneous emission from an EDFA
could be injected in the waveguide which is normally connected to the output SOA. The spectra
shown in Fig. 4b of the filtered spontaneous emission at the output of the waveguides normally
connected to the wavelength selection SOA’ ere detected with an optical spectrum analyzer (OSA).
The arm length difference L1-L2for the me
ed channel was estimated from the chip layout to be
0.73 mm which should determine a FSR of approx. 59.56 GHz. Experimentally this could be verified
by measurement the longitudinal mode spacing of each subcavity separately. Thefefore the output
signal was analyzed after O/E conversion with an electrical spectrum analyzer showing the beat signal
of the multi-longitudinal mode spectrum as an equally spaced line spectrum in the electrical domain.
W e find for each subcavity a longitudinal mode spacing of 5.276 GHz and 4.839 GHz, respectively,
leading to a FSR of -58.4 GHz.
Side mode suppression operation of the filter can be demonstrated by measuring the optical output
spectra when running each mode in the subcavities alone and then simultaneously. When the output
SOA and wavelength selection SOA#1 were switched on, whereas wavelength selection SOA#2 was
turned off, the multimode spectrum (1) shown in fig. 4a was detected with an OSA. When instead of
SOA#l, SOA#2 was active the multimode spectrum (2) was visible. If both wavelength selection
SOA’s were switched on, along with the output SOA, a singlemode spectrum (3) appeared. The
measured side mode suppression (SMS) was larger than 23 dB. We believe that this can be improved
even further. As visible in the spectra, the common mode does not lie in the transmission maximum of
either of the bandpass filters, but on the filter flank of each. By improving the design accuracy of the
WGR a better passband overlap should be reachable resulting in a stronger coupling of the subcavity
modes and an improved SMS. From Fig. 4b a spacing between the two peaks of the passbands of -38
GHz is estimated.
5 w
625 G H z l div
Res = 125GHz
1 2 5 GHz I div
k s . = 12.5 GHz
a) Optical output spectra of the DiDomenico-Seidel MFL dependent on the active SOA’s. Passbands
of the WGR (b). Absolute wavelength sifts between peaks in (a) and the corresponding maxima in (b) are
caused by different chip temperatures.
The cavity length corresponds to a longitudinal mode spacing of approximately 5 GHz indicating that
this laser should be suitable for direct high speed modulation. Each SOA’s current has to be higher
than 10 mA to 15 mA so that laser oscillation can start. Output powers higher than 0 dBm are
reachable by moderate SOA (75 mA) currents. Tuning of the SOA currents could be done over more
than 20 mA without mode hopping. The total chip size of approximately 1lmm2 shows a significant
size reduction compared to typical MFL’s. The chip size can be decreased even further.
[l] B. Glance et. al., J. Lightwave Technol. 12,957-962,1994.
[2] M. Zirngibl, IEEE Communications Magazine, pp. 39-41, 1998.
[3] C.R. Doerr, IEEE Photon. Technol. Lett. 9, 1457-1459, 1997.
[4] M. DiDomenico, IEEE J. Quantum Electronics, QE-2, 31 1-322, 1966.
[5] C.R. Doerr et al., IEEE Photon. Technol. Lett. 9, pp. 625-627, 1997
[6] C.R. Doerr, IEEE Photon. Technol. Lett. 10. pp. 528-530, 1998.
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