Wavelength-tunable semiconductor laser diode

Details Wavelength-tunable semiconductor laser diode Wavelength-tunable semiconductor laser diode

2001-01-31

2002-10-01

Leung, Quyen (Department: 2828)

Coherent light generators

Particular beam control device

Tuning

C372S045013

Reexamination Certificate

active

06459709

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to the field of wavelength-tunable semiconductor lasers, and in particular to controllably tuning the lasing wavelength by controlling the optical output power of the laser.
BACKGROUND OF THE INVENTION
Wavelength-tunable lasers have found important applications in optical communication and sensing. Wavelength-tunable lasers play a central role in particular for dense wavelength division multiplexing (DWDM) systems that form the backbone of today's optical communication network. The term “wavelength-tunable laser” is typically applied to a laser diode whose wavelength can be varied in a controlled manner while operating at a fixed heat sink temperature. At the 1550 nm wavelength regime on which most DWDM systems operate, a wavelength shift of 0.1 nm corresponds to a frequency shift of about 12.6 GHz. At a given heat sink temperature, the central wavelength of a conventional distributed feedback (DFB) laser diode may be red-shifted by as much as 0.3 nm or about 40 GHz due to the rise in the temperature of the junction by Ohmic losses. In contrast, at a given heat sink temperature, the wavelength of a tunable laser may vary by several nanometers, corresponding to hundreds or even thousands of GHz, covering several wavelength channels on the International Telecommunication Union (ITU) grid. Depending on the physical mechanisms of wavelength tuning, the lasing wavelength can be tuned in either positive (red) or negative (blue) direction. Controlled wavelength tunability offers many advantages over conventional fixed wavelength DFB lasers for DWDM operation. It enables advanced all-optical communication networks as opposed to today's network where optics is mainly used for transmission and the network intelligence is performed in the electronic domain. All-optical networks can eliminate unnecessary E/O and O/E transitions and electronic speed bottlenecks to potentially achieve very significant performance and cost benefits. In addition, a less extensive inventory of wavelength-tunable lasers than of laser with a fixed wavelength is required. Keeping a large inventory of lasers for each and every wavelength channel can become a major cost issue. For advanced DWDM systems, the channel spacing can be as narrow as 50 GHz (or about 0.4 nm in wavelength), with as many as 200 optical channels occupying a wavelength range of about 80 nm. For the reasons stated above, wavelength-tunable lasers have attracted considerable interest in optoelectronic device research.
There exist different design principles for tunable lasers. Almost all wavelength-tunable laser designs make use of either the change of refractive indices of semiconductor or the change of laser cavity length to achieve wavelength tuning. For the former, common mechanisms for index change include thermal tuning, carrier density tuning (a combination of plasma effect, band-filling effect, and bandgap shrinkage effect), electro-optic tuning (linear or quadratic effect), and electrorefractive tuning (Franz-Keldysh or quantum confined Stark effect). For DFB lasers, the wavelength of the laser light propagating in the waveguide is basically determined by the grating period &Lgr;. The free-space lasing wavelength &lgr; is given by &lgr;=2 n
eff
&Lgr;, where n
eff
is the effective index of refraction of the waveguide and &Lgr; is the period for first-order gratings. Accordingly, the change &Dgr;&lgr; in the lasing wavelength &lgr; is directly proportional to the change &Dgr;n of the index of refraction n
eff
.
Referring to
FIG. 1
, a prior art three-section DBR tunable laser
100
includes an optical gain section
101
, a phase control section
102
, and a tunable DBR section
103
. A first current source
104
pumps the gain section
102
to generate optical gain; a second current source
105
injects carriers to adjust the phase condition of the phase control section
102
so that the resonant frequency matches approximately the peak of the DBR reflectivity; and a third current source
106
controls the reflectivity peak by changing the effective index n
eff
of the Bragg waveguide section
103
. With proper selection of the currents in the DBR region
103
and in the phase control region
102
, quasi-continuous wavelength tuning can be achieved. All three sections
101
,
102
,
103
are optically connected to minimize residue reflections and coupling loss; however, the sections
101
,
102
,
103
have to be electrically isolated from one another, for example, by layers
107
disposed between the respective sections
101
,
102
,
103
. Three currents, responsible for the gain region, DBR region, and phase control region, have to be supplied; and the lasing wavelength depends on all three currents and is particularly sensitive to the currents in the DBR and phase control region. A continuous wavelength tuning range of about 10 nm can be achieved using this design.
Modifications of the three-section DBR lasers include sampled grated four-section DBR lasers and vernier-tuning sampled grating DBR lasers (not shown). The last device requires four separately controlled current sources to achieve the full tuning range (about 80 nm quasi-continuous tuning).
Alternatively, the lasing wavelength can also be changed by changing the physical length of laser cavity in the surface normal direction. This mechanism has been applied, for example, to vertical-cavity surface-emitting lasers (VCSELs) where typically due to the short cavity length only one or at most very few lasing modes fall within the gain peak. Referring to
FIG. 2
, a prior art wavelength-tunable VCSEL structure
200
is based on surface micromachining technology. The laser device
200
includes a bottom dielectric DBR mirror
202
, a top dielectric DBR mirror
201
, an electrostatically controlled membrane
203
, and an active region
204
. Electrically pumped micro-electro-mechanically tuned VCSEL in the 1550 nm wavelength regime have not yet been demonstrated. However, the laser device
200
can be optically pumped by an incoming pump beam
205
(e.g. a beam from a 980 nm wavelength pump laser) through the bottom mirror
202
, with the laser output
206
being emitted from the top mirror
201
disposed on the membrane
203
. Wavelength-tuning is obtained by changing the cavity length of the VCSEL through the movement of the membrane
203
. With a surface micromachined tunable mirror, a continuous tuning range of 40 nm has been demonstrated with an output power of up to 7 mW coupled to a single mode fiber.
Multiple-section DFB lasers in general have a smaller tuning range than multiple-section DBR lasers, except for the tunable twin-guide (TTG) DFB lasers where relatively wide (about 6 nm) and continuous tuning can be achieved.
In DWDM systems, the wavelength of the channel has to be stabilized within a few gigahertz from the ITU grid, typically less than 10% of the channel spacing. A change of the junction temperature and/or device degradation can cause wavelength drift beyond its acceptable range. Achieving wavelength stability requires monitoring the wavelength in real time using a sophisticated feedback mechanism. Several commercially available devices and their operation for accurately monitoring the laser emission wavelength are shown in
FIGS. 3
,
4
, and
5
. Common to these devices is an optical interference device such as a Fabry-Perot etalon placed between the laser and a photodetector. Critical for the device performance are the mechanical stability and angular precision of the etalon and the collimation of the laser beam impinging on the etalon.
Referring now to
FIG. 3
, a wavelength-monitoring system
300
includes an optical beam splitter
301
, a Fabry-Perot (F-P) etalon
302
connected to a first output of the beam splitter
301
, a first photodetector (PD)
303
following the F-P etalon
302
, with a second PD
304
connected to the second output of the beam splitter
301
as a reference detector. Once the system is calibrated, the lasing wavelength can be determined from the ratio of the photo

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