Wavelength-tunable lasers

Information

  • Patent Grant
  • 6693946
  • Patent Number
    6,693,946
  • Date Filed
    Tuesday, August 14, 2001
    23 years ago
  • Date Issued
    Tuesday, February 17, 2004
    20 years ago
Abstract
A semiconductor laser includes first and second laser cavities. The first and second cavities share a common optical gain medium and lase at different wavelengths.
Description




BACKGROUND OF THE INVENTION




1. Field of the Invention




The invention relates to lasers.




2. Discussion of the Related Art




In optical communications, the use of wavelength division multiplexing (WDM) to enable high bandwidth communication is expanding. A WDM system simultaneously transmits several distinct communications over one fiber by transmitting the communications in different wavelength channels. The wavelength channels are typically closely spaced to more efficiently use wavelength ranges for which the optical fiber has better transmission properties, e.g., lower attenuation.




The need for low cost and closely spaced wavelength channels poses challenges to the development of WDM systems. In particular, WDM transmitters have to produce output wavelengths that do not drift between the different wavelength channels. Unfortunately, the output wavelengths of the semiconductor lasers typically used in WDM transmitters tend to drift unless complex and expensive temperature controls are added. The drift results from temperature variations in laser cavities that lead to changes in the optical properties of the semiconductor medium therein.




Another problem in WDMs relates to the need for multi-chromatic transmitters. Such transmitters are able to transmit light in multiple wavelength channels of the WDM system concurrently. Disadvantageously, multi-chromatic transmitters are usually more complex and high cost.




SUMMARY OF THE INVENTION




A multi-chromatic laser embodying the principles of the invention features multiple laser cavities that share a common gain medium. The laser may include several wavelength-selective reflectors with different characteristic reflection wavelengths as an advantageous way of enabling the multiple cavities to share the common gain medium. Such a laser can be used, for example, as a multi-chromatic light source for an optical transmitter in a WDM or other optical network. Such transmitters are typically less complex than prior art multi-chromatic optical transmitters.




One embodiment of the invention features a so-called external cavity laser in which the laser cavity includes an internal waveguide and an external waveguide that is serially connected to the internal waveguide. The external waveguide has cascaded first and second wavelength-selective reflectors for reflecting first and second wavelengths, respectively. The first and second reflectors define the multiple laser cavities.











BRIEF DESCRIPTION OF THE DRAWINGS





FIG. 1

shows an external cavity laser that embodies the principles of the invention by having two laser cavities that share a common optical gain medium;





FIG. 2



a


is a graph illustrating the spectral reflectivity of a generic Bragg grating;





FIG. 2



b


shows reflectivities of the two Bragg gratings used in an external cavity laser of the type shown in

FIG. 1

;





FIG. 3

is a time line illustrating a particular pump current in the laser's gain medium and associated lasing wavelengths;





FIG. 4

shows another external cavity laser that uses an optical circulator to enable sharing of one optical gain medium by two laser cavities;





FIG. 5

shows another external cavity laser that uses a Y-coupler to enable two laser cavities to share one gain medium;





FIG. 6

shows another external cavity laser in which more than two laser cavities share a common gain medium; and





FIG. 7

shows a multi-chromatic optical transmitter for a WDM system that is based on one of the lasers of

FIGS. 1

,


4


,


5


, or


6


.











In the Figures, like reference numbers indicate like elements.




DETAILED DESCRIPTION OF THE INVENTION




Several approaches have been tried for economically stabilizing the output wavelengths of such lasers. One approach to temperature stabilization involves linearly coupling an external fiber to one output of a semiconductor optical amplifier. The amplifier includes an internal optical waveguide that provides optical gain when pumped. The internal waveguide couples at one end to the output of the amplifier and at a second end to a reflector. The external fiber forms a linear extension of the internal optical waveguide. The external fiber also contains an external Bragg grating. The Bragg grating reflects light of a particular wavelength and thereby provides the optical feedback needed to close a laser optical cavity that is formed by the fiber and the internal optical waveguide. Positioning the Bragg grating external to the optical gain medium makes the lasing wavelength less sensitive to temperature variations in general because of the high temperature-stability of silica-glass gratings.




Lasers in which a part of a laser cavity is external to a semiconductor structure housing the laser's optical gain medium are generally referred to as external cavity lasers.





FIG. 1

shows an external cavity laser


10


that is capable of simultaneously lasing at two wavelengths. The laser


10


includes a monolithic semiconductor structure


12


and an external silica-glass waveguide


14


, e.g., an optical fiber or planar waveguide. The semiconductor structure


12


includes an internal semiconductor waveguide


16


having one end that is terminated by a reflector


18


, e.g., a reflective layer or a cleaved facet. The other end of the waveguide


16


serially couples to the silica-glass waveguide


14


. The silica-glass waveguide


14


includes first and second Bragg gratings


20


,


22


with different line spacings. Exemplary Bragg gratings


20


,


22


include gratings in the waveguide


14


and waveguides external to the waveguide


14


. The first and second Bragg reflectors


20


,


22


and the reflector


18


form end reflectors of two optical Fabry-Perot cavities


24


,


26


that share a common optical gain medium in semiconductor waveguide


16


.




Bragg gratings


20


and


22


are optical reflectors that reflect light whose wavelengths are an integer multiple of the gratings' characteristic reflection wavelengths λ


B1


and λ


B2


, respectively.





FIG. 2A

shows the reflectivity of a generic Bragg grating. The Bragg grating has a large reflectivity at a characteristic reflection wavelength λ


B


and at integral multiples of the wavelength λ


B


. The Bragg grating only very weakly reflects light at other wavelengths. A Bragg grating's characteristic reflection wavelength is λ


B


≡2 nL where L is the repeat period for grating features and “n” is the index of refraction.




Referring to

FIG. 2B

, cascaded Bragg gratings


20


,


22


of

FIG. 1

have different periods, i.e., L


1


≠L


2


. Thus, these Brag gratings


20


,


22


have different characteristic reflection wavelengths, i.e., λ


B1


≠λ


B2


. Thus, the reflectivities R


1


, R


2


of the two Bragg gratings


20


,


22


do not have high values for the same wavelengths. Bragg gratings


20


,


22


do not reflect back significant amounts of light at the same wavelengths.




Referring to

FIG. 1

, since the two Bragg gratings


20


,


22


do not reflect significant amounts of light at the same wavelengths, the two optical cavities


24


,


26


lase at different wavelengths. Thus, laser


10


lases at two different wavelengths, e.g., λ


1


and λ


2


, and can simultaneously produce light of both wavelengths at optical output


28


. Since the characteristic reflection wavelengths are fixed by the gratings' periods L


1


and L


2


, the two wavelengths, λ


1


and λ


2


can have close values, e.g., neighboring wavelength channels of a WDM system, if the two grating periods for Bragg gratings


20


and


22


are appropriately selected.




Still referring to

FIG. 1

, semiconductor waveguide


16


includes a core


30


and guiding layers


31


. The core


30


includes the optical gain medium that generates light spontaneously in response to being current-pumped via metal contacts


32


,


34


. The guiding layers


31


have a lower refractive index than the core


30


and thus, strongly guide light to propagate along the longitudinal axis of the core


30


.




In exemplary lasers


10


, structure


12


is a composite of doped semiconductor layers, e.g., InGaAsP or another group III-V semiconductor. Some such semiconductor structures are disclosed in U.S. Pat. Nos. 5,574,742 and 5,870,417, which are incorporated by reference in their entirety. Other constructions of semiconductor structure


12


are well-known in the art.




To improve the optical coupling between the internal and external waveguides


16


,


14


, structure


12


includes a beam expander/contractor region


36


and an anti-reflection layer


38


.




The beam expander/contractor region


36


expands or contracts the diameter of the fundamental propagation mode in the internal waveguide


16


to match the diameter of the fundamental propagation mode in the external waveguide


14


. This matching of the diameters improves the optical coupling between the internal and external waveguides


16


,


14


and decreases the dependence of the coupling on the precision of the alignment between the waveguides


14


,


16


at surface


40


. Beam expanders/contractors and antireflection layers are further described in co-pending U.S. patent application Ser. No. 09/608,639, filed Jun. 30, 2000, commonly assigned, which is incorporated herein by reference in its entirety.




The anti-reflection layer


38


suppresses back reflections of light from surface


40


. Reducing back reflections at surface


40


improves the coupling between internal and external waveguides


16


,


14


. The absence of back reflections at surface


40


also insures that internal waveguide


16


does not function as a Fabry-Perot cavity that is closed by reflector


18


and surface


40


. The Bragg gratings


20


,


22


stabilize the output wavelengths of cavities


24


,


26


from such temperature fluctuations, because the output laser wavelengths are fixed to the characteristic reflection wavelengths of the gratings


20


,


22


.




One end of external waveguide


14


is physically fixed adjacent end surface


40


so that light exiting internal waveguide


16


via beam expander/contractor


36


enters into the waveguide


14


. Using the beam expander/contractor


36


eliminates the need for other coupling optics between the two waveguides


14


,


16


. The light exiting the internal waveguide


16


is simply directed into the polished end of the external waveguide


14


. Exemplary waveguides


14


include an optical fiber positioned in a V-groove (not shown) to be aligned normal to the end surface


40


and a planar waveguide positioned normal to the end surface


40


. In some embodiments, an index-matching medium is placed between the waveguide


14


and the structure


12


to further reduce reflections at the end surface


40


.




In exemplary embodiments, the two Bragg gratings


20


,


22


provide different reflectivities. Exemplary maximum reflectivities are between about 0.50 and about 0.95.





FIG. 3

is a graph


40


of a particular time-dependent pump current applied between terminals


32


,


34


of laser


10


, shown in FIG.


1


.

FIG. 3

also shows the corresponding optical response for the various pump currents. Below a threshold pump current


44


, the laser


10


produces only spontaneous emission light, i.e., no sustained stimulated emission (S.S.E.), because cavity losses surpass the power provided by the pumping current. At a higher pump current


46


, the laser


10


lases at wavelength λ


1


, because the propagation loss rates are lower than the pumping power for wavelength λ


1


. At a higher pump current


48


, the laser


10


lases at both wavelength λ


1


and wavelength λ


2


, because the pumping power is larger than average propagation loss rates for both wavelengths λ


1


and λ


2


.




Combined losses in waveguide


14


,


16


control output wavelengths of laser


10


. If losses for a wavelength are too large, lasing does not occur at that wavelength. Some embodiments introduce variable losses to stop lasing at λ


1


for pump currents


40


,


48


of FIG.


3


. The variable losses may be in internal waveguide


16


or in external waveguide


14


. For example, a spectrally dependent beam splitter can be used to eliminate optical feedback at wavelength λ


1


when the laser


10


is pumped with currents


40


,


48


. Then, the laser


10


lases only at wavelength λ


2


for the currents


40


,


48


.




Various geometrical arrangements of Bragg gratings produce other external cavity lasers with advantageous features.





FIG. 4

shows an alternate external cavity laser


10


′. In the laser


10


′, external waveguide


14


′ includes waveguide arms A-C and optical circulator


52


. The optical circulator


52


directs light from waveguide arm A to waveguide arm B, from waveguide arm B to waveguide arm C, and from waveguide arm C back to waveguide arm A. The waveguide arm A includes a Bragg grating


20


whose characteristic reflection wavelength is λ


B1


. The waveguide arms B and C include Bragg gratings


22


′,


22


″, which have equal characteristic reflection wavelengths λ


B2


.




Laser


10


′ is able to simultaneously lase at wavelengths λ


1


and λ


2


, because Bragg grating


20


has a different characteristic reflection wavelength than gratings


22


′,


22


″. Lasing at wavelength λ


1


occurs in cavity


24


′ whose external includes a portion of waveguide arm A and grating


20


. Lasing at wavelength λ


2


occurs in a cavity


26


′ whose external portion includes waveguide arms A-C, circulator


52


, and Bragg gratings


22


′,


22


″.




In laser


10


′, Bragg grating


22


′ is a high quality reflector, e.g., reflecting 95%-99% of the incident light of wavelength λ


2


. Due to the selective high reflectivity, the grating


22


′ reflects almost all light of wavelength λ


2


back to the optical circulator


52


, and thus, output port


54


transmits almost exclusively light at wavelength λ


1


. Even when the laser


10


′ lases at both wavelengths λ


1


and λ


2


, about 95-99% of the light intensity at port


54


is at wavelength λ


1


.




In laser


10


′, Bragg grating


22


″ is a relatively lower quality reflector, e.g., reflecting 50%-80% of the incident light of wavelength λ


2


Due to the lower reflectivity, output port


56


transmits light at wavelength λ


2


with a significant intensity when the laser


10


′ lases at both wavelengths λ


1


and λ


2


. The output port


56


does not transmit significant amounts of light at wavelength λ


1


, because waveguide arm C only transmits light that is incident onto optical circulator


52


from waveguide arm B. Very little light at wavelength is reflected by Bragg grating


22


′ back to the circulator


52


via waveguide arm B.




Thus, optical circulator


52


enables external cavity laser


10


′ to produce substantially monochromatic output light at ports


54


and


56


even when simultaneously lasing at two wavelengths λ


1


and λ


2


.





FIG. 5

shows another external cavity laser


10


″. In the laser


10


″, external waveguide


14


″ includes optical fiber arms A′-C′ and a fiber Y-coupler


62


, which directs light from fiber arm A′ to both fiber arms B′ and C′. The fiber arms B, C′ include Bragg grating


20


″ and


22


″, which reflect light at wavelengths λ


1


and λ


2


, respectively. Both gratings


20


″ and


22


″ are high quality reflectors, e.g., reflecting 95%-99% of incident light at respective peak wavelengths λ


1


and λ


2


. Due to the high reflectivity of gratings


20


″ and


22


″, output port


64


and output port


66


transmit almost exclusively light at wavelength λ


2


and λ


1


respectively. For example, more than 95% of the intensity at each port


64


,


66


is only of one wavelength even when the laser


10


″ lases at both wavelengths λ


1


and λ


2


.




Those of skill in the art know how to construct fiber Y-coupler


62


. The construction typically includes partially denuding segments of two optical fibers of cladding layers and positioning the two denuded segments adjacent and parallel to each other. The construction produces a device having four coupled fiber arms A′-D′. Fiber arm D′ serves as a monitoring arm for optical intensities lasing in the laser


10


″.




Exemplary external cavity lasers are able to simultaneously lase at more than two wavelengths.





FIG. 6

shows an external cavity laser


10


″″ that can lase at wavelengths λ


1


, λ


2


, and λ


3


in response to appropriate pump currents. In the laser


10


″″, an external optical fiber


14


′″ includes three Bragg gratings


20


′″,


22


′″,


23


′″ with different characteristic reflection wavelengths. Thus, Bragg gratings


20


′″,


22


′″,


23


′″ form laser cavities


77


-


79


that sustain stimulated emission at a different wavelength λ


1


, λ


2


and λ


3


.




Various embodiments use external cavity lasers


10


,


10


′,


10


″,


10


″″ of

FIGS. 1

,


4


-


6


as light sources in transmitters for wavelength division multiplexed (WDM) optical networks. The lasers


10


,


10


′,


10


″,


10


″″ produce output light for more than one wavelength channel of the WDM network based on a single pumped gain medium. Thus, using these lasers


10


,


10


′,


10


″,


10


″″ can simplify transmitter designs and reduce transmitter costs in WDM networks.





FIG. 7

shows a transmitter


80


for a WDM network. The transmitter


80


includes an external cavity laser


82


adapted to simultaneously lase at wavelengths λ


1


and λ


2


when properly pumped, e.g., laser


10


′ of FIG.


4


. The laser


82


produces output light of wavelength λ


1


and λ


2


at output port


84


and output port


86


, respectively. Optical fibers couple the ports


84


,


86


to input ports of optical intensity modulators


88


,


90


, e.g., electrically controlled attenuators. Optical fibers couple output ports of the intensity modulators


88


,


90


to a 2×1 fiber combiner


92


that couples the transmitter


80


to a transmission fiber


94


of the WDM network.




The wavelengths λ


1


and λ


2


correspond to two separate wavelength channels of the WDM network. Thus, intensity modulators


88


,


90


can be independently controlled so that transmitter


80


transmits data simultaneously on two wavelength channels of the WDM system.




Some embodiments of WDM transmitters produce output optical signals from external cavity lasers described herein by modulating pump currents in the shared gain medium of the lasers.




Other embodiments of the invention will be apparent to those skilled in the art in light of the specification, drawings, and claims disclosed herein.



Claims
  • 1. A laser, comprising:an external cavity laser having first and second laser cavities, the first and second cavities sharing a common optical gain medium and being constructed to lase at different wavelengths; and an optical circulator; wherein the first laser cavity includes a first grating; the second laser cavity includes a second grating; and the first and second gratings have different characteristic reflection wavelengths; and wherein one of the cavities includes portions of multiple arms of the optical circulator.
  • 2. The laser of claim 1, whereinthe first laser cavity further comprises a first silica-glass waveguide and a semiconductor waveguide that includes the gain medium; and the second laser cavity further comprises a second silica-glass waveguide and the semiconductor waveguide.
  • 3. The laser of claim 2, wherein each silica-glass waveguide includes one or more optical fibers.
  • 4. The laser of claim 2, wherein each silica-glass waveguide includes one or more planar waveguides.
  • 5. The laser of claim 1, wherein the first and second gratings are serially cascaded.
  • 6. The laser of claim 1, wherein the first and second gratings are Located on different arms of the optical circulator.
  • 7. The laser of claim 6, further comprising a third grating located on a different arm of the optical circulator than the first and second gratings, the third grating configured to reflect light at the same characteristic wavelength as the second grating.
  • 8. A laser, comprising:a laser cavity capable of generating laser light, the laser cavity having an internal semiconductor waveguide and an external portion including a silica-glass waveguide, the waveguides being linearly coupled, the silica-glass waveguide being coupled to cascaded first and second wavelength-selective reflectors that reflect light at first and second lasing wavelengths, respectively; and an optical circulator; and wherein the first and second reflectors are Bragg gratings with first and second periods, respectively; and wherein the external portion includes a portion of the optical circulator between two arms of the optical circulator.
  • 9. The laser of claim 8, wherein the silica-glass waveguide includes one or more optical fibers.
  • 10. The laser of claim 9, wherein one of the gratings is located in the one or more optical fibers.
  • 11. The laser of claim 8, wherein the internal semiconductor waveguide includes a pumpable optical gain medium.
  • 12. The laser of claim 11, further comprising:a reflector positioned to reflect light incident from the internal semiconductor waveguide.
  • 13. The laser of claim 8, wherein the first and second gratings are located on different arms of the optical circulator.
  • 14. The laser of claim 13, further comprising a third grating located on a different arm of the optical circulator than the first and second gratings, the third grating configured to reflect light at the same characteristic wavelength as the second grating.
  • 15. An optical transmitter, comprising:an external cavity laser having first and second laser cavities, the first and second cavities sharing a single optical gain medium and being constructed to lase at least at first and second wavelengths; and a modulator coupled to the laser and capable of modulating output intensities of the laser at the first and second wavelengths; an optical circulator; and wherein the laser further comprises a semiconductor optical waveguide that includes the optical gain medium; and wherein the first cavity includes a first silica-glass waveguide, the semiconductor optical waveguide, and a first grating and the second cavity includes a second silica-glass waveguide, the semiconductor optical waveguide, and a second grating; and wherein the first and second gratings having different characteristic reflection wavelengths and the second cavity includes portions of multiple arms of the optical circulator.
  • 16. The optical transmitter of claim 15, wherein the modulator includes first and second controllable attenuators coupled to first and second output ports of the laser, the first and second ports configured to output laser light at the first and second wavelengths, respectively.
  • 17. The optical transmitter of claim 15, wherein the silica-glass waveguides include portions of one or more optical fibers.
  • 18. The optical transmitter of claim 15, wherein the silica-glass waveguides include portions of one or more planar waveguides.
  • 19. The laser of claim 15, wherein the first and second gratings are located on different arms of the optical circulator.
  • 20. The laser of claim 19, further comprising a third grating located on a different arm of the optical circulator than the first and second gratings, the third grating configured to reflect light at substantially the same characteristic wavelength as the second grating.
Parent Case Info

This application claims the benefit of U.S. Provisional Patent Application No. 60/303,259, filed Jul. 5, 2001.

US Referenced Citations (6)
Number Name Date Kind
4689065 Krause Aug 1987 A
5574742 Ben-Michael et al. Nov 1996 A
5870417 Verdiell et al. Feb 1999 A
6041070 Koch et al. Mar 2000 A
6320888 Tanaka et al. Nov 2001 B1
6389047 Fischer May 2002 B1
Foreign Referenced Citations (1)
Number Date Country
2000194023 Jul 2000 JP
Non-Patent Literature Citations (1)
Entry
U.S. patent application Ser. No. 09/608,639, Chen et al., filed Jun. 30, 2000.
Provisional Applications (1)
Number Date Country
60/303259 Jul 2001 US