WO2001035505A1

WO2001035505A1 - High resolution wavelength locking technique for one- and two-dimensional arrays of semiconductor diode lasers

Info

Publication number: WO2001035505A1
Application number: PCT/US2000/042046
Authority: WO
Inventors: Andrew D. Sappey
Original assignee: Zolo Technologies, Inc.
Priority date: 1999-11-10
Filing date: 2000-11-09
Publication date: 2001-05-17
Also published as: TW480798B; AU2923701A

Abstract

An external cavity semiconductor laser provided with a semiconductor laser diode array (40), a diffraction grating (32), and a mirror (34). The diode laser array (40) is positioned to illuminate the diffraction grating (32) and the mirror (34) to reflect selected wavelengths back into the cavity for plural wavelength locked beams with a single resonant cavity. The laser array (40) provides lasers (36, 38) at different spatial positions of the resonant cavity (30) for resonating laser beams at multiple wavelengths providing a cost effective approach for generating multiple wavelengths. A first semiconductor diode laser (36) of the array illuminates the diffraction grating with a first beam at a first angle of incidence, and a second semiconductor diode laser (38) of the array illuminates the diffraction grating with a second beam at a second angle of incidence. A mirror (34) positioned opposite the diffraction grating reflects the first beam (42) and the second beam (44) deflected from the diffraction grating (32) to provide a first select order of reflection from said diffraction grating back to the first laser (36) and the second laser (38) respectively. An output (46) couples a second select order of reflections from the diffraction grating (32). A modified Littman-Metcalf configuration is adapted to the implementation of a single external cavity to minimize the costs associated with multiple external cavity diode lasers and/or wavelength lockers for each of multiple semiconductor diode lasers. The described embodiments provide innovative applications of one- and two-dimensional, position-dependent wavelength feedback to lock arrays of semiconductor lasers at a particular, well-controlled wavelength, using the best attributes of crossed dispersion techniques (high resolution and two-dimensional dispersion) to achieve this goal.

Description

HIGH RESOLUTION WAVELENGTH LOCKING

TECHNIQUE FOR ONE- AND TWO-DIMENSIONAL

ARRAYS OF SEMICONDUCTOR DIODE LASERS

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to optical communications and particularly to improved systems and methods for fiber-optic communications requiring wavelength locking of multiple emitters from arrays of semiconductor diode lasers for providing multiple wavelengths (λs) from a single source. The invention is intended for use as emitter technology for wavelength division multiplexed (WDM) applications within the fiber optic telecommunications industry.

2. Description of the Related Art Wavelength tunable semiconductor diode lasers for use in wavelength division multiplexed (WDM) optical communication systems are known. A specific arrangement of laser, grating, and mirror called Littman-Metcalf configuration is applicable to the implementation of external cavities in which the diffracted beam is directed back toward the laser. Such tunable semiconductor diode lasers provide useful optical tools but they are very expensive and must be locked individually. Additionally, tunable external cavity semiconductor diode lasers are typically used in testing and measurement applications, and thus may not be capable of generating all desired wavelengths within a single cavity structure for a cornmunications system.

Currently, distributed feedback (DFB) lasers are used to produce light for fiber optic communications. However, DFB lasers without external feedback control have a lasing bandwidth that drifts on the order of 0.5 ran, which is too broad for next generation ultra-high density wavelength division multiplexing (DWDM) systems that require a lasing bandwidth on the order of 0.05 ran or less. Wavelength lockers may be used to lock the generated wavelengths with respect to each other, however applications requiring multiple wavelengths become costly due to the corresponding need for multiple wavelength lockers. While systems employing multiple external cavity diode laser systems may be used, these systems are also very expensive (> $10,000 for a single wavelength), and for new ultra-DWDM applications, hundreds of external cavity diode lasers would be required.

In telecommunications diode laser systems, the active lasers are wavelength controlled by distributed feedback from a monolithic grating that is located on the laser chip, which has certain inherent limitations. In order to reduce the lasing bandwidth of these devices to approximately 0.05 ran to 0.1 ran for modern applications that necessitate such requirements, external cavity diode laser (ECDL) systems have been developed. Typically the desired wavelength is selected in a conventional prior art Littman-Metcalf ECDL configuration with a movable mirror which selects a desired wavelength from the beam diffracted by a diffraction grating. Moving parts for variation in the angle of the mirror by translation and/ or rotation effectively selects the desired wavelength diffracted by the grating at the angle represented by the mirror position. Accordingly, it would be desirable to provide methods and systems for generating multiple wavelengths for optical communication systems that are reliable and avoid the moving parts associated with known ECDL resonating cavities to achieve multiple wavelengths. Additionally, it would be desirable to utilize a single resonating cavity to minimize the costs associated with multiple external resonating cavity and/ or wavelength lockers for each of multiple semiconductor diode lasers. To this end, it would be advantageous to provide lasers that facilitate differing spatial positions within a single external cavity for resonating several laser beams at multiple wavelengths. By providing a cost effective approach for generating multiple wavelengths, it is believed that DWDM systems may be achieved for network applications such as metropolitan area networks, which otherwise may be cost prohibitive using prior art techniques.

SUMMARY OF THE INVENTION The present invention facilitates the ability to produce a large number of independently controlled, wavelength-locked semiconductor diode lasers for telecorrtmunications applications by providing plural lasers of a laser array at different spatial positions within a single external cavity resonator without the use of moving parts. The described embodiments dramatically reduce the cost of laser systems designed to produce a large number of highly resolved, but closely spaced wavelengths (λs) by locking the wavelength of each laser emitter through feedback using a massively parallel feedback technique while minimizing drift. The methods and systems described utilize crossed dispersion in a modified Littman-Metcalf configuration with an external laser cavity to provide position dependent, multi-wavelength feedback to a laser emitter array, either one-dimensional or two-dimensional. The feedback locks the laser wavelength to the feedback wavelength causing each laser in the array to emit at a well-defined and stable wavelength. This, in turn, allows a large number of emitters to be utilized simultaneously. Such cost-effective technology may be critical to successful development of ultra-high density wavelength division multiplexing (DWDM) systems that may have a large number of channels, thus requiring very narrow lasing bandwidths. Briefly summarized, the invention relates to an external cavity semiconductor laser provided with a semiconductor laser diode array, a diffraction grating, and a mirror. The diode laser array is positioned to illuminate the diffraction grating and the mirror to reflect selected wavelengths back into the cavity for plural wavelength locked beams with a single resonant cavity. A first semiconductor diode laser of the array illuminates the diffraction grating with a first beam at a first angle of incidence, and a second semiconductor diode laser of the array illuminates the diffraction grating with a second beam at a second angle of incidence. A reflector positioned opposite the diffraction grating reflects the first beam and the second beam deflected from the diffraction grating to provide a first select order of reflection, preferably the first-order reflection from said diffraction grating back to the first laser and the second laser respectively. An output couples a second select order, preferably the zeroth-order, reflections from the diffraction grating. The configuration is adapted for the implementation of a single external cavity in which multiple wavelength diffracted beams are directed back toward the laser to minimize the costs associated with multiple external cavity diode lasers and/ or wavelength lockers for each of multiple semiconductor diode lasers. The laser array provides lasers at different spatial positions of the resonant cavity for resonating laser beams at multiple wavelengths providing a cost effective approach for generating multiple wavelengths.

The described embodiments provide one- and two-dimensional, position-dependent wavelength feedback to lock arrays of semiconductor lasers at a particular, well-controlled wavelength, using the high resolution and two- dimensional dispersion attributes of crossed dispersion techniques. Therefore, it is an object of the invention to provide improved methods and systems for wavelength locking of one- and two-dimensional arrays of semiconductor lasers. Other objects, advantages, and novel features will be apparent from the following description of the present preferred embodiments when read in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of a conventional Littman-Metcalf configuration external cavity diode laser resonator;

FIG. 2 is a schematic diagram of a resonator in accordance with the present invention used to wavelength lock a one-dimensional laser diode array using a modified Lit±tnan-Metcalf configuration; and

FIG. 3 is a schematic diagram of an alternate embodiment of a resonator in accordance with the present invention employing a design to wavelength lock a two-dimensional array of semiconductor lasers.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a schematic diagram of a conventional Littman-Metcalf external cavity semiconductor diode laser resonator system 10. The diode lasers typically incorporate a high-reflection-coated rear facet, a lasing medium, an anti-reflection-coated front facet, a window of the diode-laser case, and a collimating lens. A wide variety of low-power/ high-speed communications diode lasers are now commercially available, including Fabry-Perot type lasers, distributed feedback (DFB), and distributed Bragg reflector (DBR) lasers available at various wavelengths. For external cavity diode lasers, Fabry-Perot type lasers with an anti-reflection coated output (or front) facet are typically used. The diode laser's gain versus wavelength curve is determined by the semiconductor material and structure, and is a function of both temperature and carrier density. Present technology is sufficient for controlling the spectral characteristics of diode lasers. In the Littman-Metcalf design, light from a single diode laser 12 is collimated and directed to a grating 22 in grazing incidence configuration. The first-order reflection 14 from this grating 22 is directed to a reflective surface 24 which is mounted on a special rotation/ translation stage 16 for rotation 18 and translation 20 provided with stage 16. The frequency is tuned by moving the external mirror 24. This, in turn, tunes the wavelength of the laser 12. The laser 12 emits a beam 26 at an angle of incidence which illuminates the grating 22. The wavelength that is fed back to the diode 12 is altered by rotating the mirror instead of the grating 22.

The basic principles of the operation of the Littman-Metcalf configuration tunable laser of FIG. 1 utilize the external cavity resonator system 10 in conjunction with the grating 22 being provided as a diffraction grating and the reflector 24 and stage 16 being a movable mirror that diffracts the laser beam at the angle normal to the mirror 24 such that the first-order reflected beam 26' is directed back onto the grating 22 and from there, back into the lasing cavity of the single diode laser 12. Thus, the first-order reflection 26' from the grating 22 is reflected back into the laser by the external mirror 24, where it serves to determine the operating wavelength of the system 10. The maximum wavelength range of an ECDL depends on the design of the cavity and on the detailed characteristics of the diode laser chip. Factors that affect the tuning include characteristics such as the facet reflectances, the degree to which the spectral broadening is homogeneous, the side-mode suppression ratio, the pumping rate, the spatial mode control, and the potential for coupling between modes. As discussed, translation and/ or rotation of the mirror 24 with the stage 16 selects the wavelength diffracted and allows the system 10 to be tuned to a desired output wavelength. The zeroth-order reflection 26" from the grating is the output 24 of the laser from the system 10. Thus, one end of the resonant cavity is the rear facet of the diode laser 12, and the other end is the external mirror 24.

The system 10 normally uses the Littman-Metcalf configuration in which the light from the single diode laser 12 is collimated and directed to a grating 22 in grazing incidence configuration. With the first-order reflection 26 from this grating being directed to the mirror 24, the special rotation translation stage 16 is necessitated to achieve desired wavelengths. This, in turn, tunes the wavelength of the laser of system 10. The length of the cavity changes by a very slight amount during the tuning to assure that the laser does not "mode hop". The feedback from the mirror 24 reflects from the grazing incidence grating in first-order and is focused into the diode to lock its wavelength. An angle, θ, between the diode output and the mirror position controls the lasing wavelength. Light is coupled out of the cavity via the zeroth-order grating reflection 26" of the laser beam.

In one embodiment, the wavelength lock is provided as a one- dimensional array of semiconductor lasers is shown in FIG. 2. The resonator shown is used to wavelength lock a one-dimensional laser diode array. Light at only two different wavelengths is ray-traced (42, 42', 42" and 44, 44', 44") through the resonator to show how the system functions. In this application, a mirror 34 is not rotated to tune the output; rather, the angle that each diode in the array makes with the mirror 34 and grating causes the feedback wavelength to change as a function of position, thereby causing the lasing wavelength of each diode to be different. With reference to the external cavity diode laser system 30 of FIG. 2, plural wavelength locked beams are provided with a single resonant cavity. A grating 32 is provided opposite a reflector 34 which may be provided as a mirror for reflecting first-order reflections 42', 44' of the plural wavelength locked beams 42, 44 of the system 30. The mirror preferably has a reflective surface coated with a suitable reflective metal such as gold or aluminum. The grating 32 is a diffraction grating in the described embodiment which may be provided as a holographic or ruled grating. A first semiconductor diode laser 36 is provided within an array of lasers 40 which also includes a second semiconductor laser diode 38. The first semiconductor laser diode 36 is provided for illuminating the diffraction grating 32 with the first beam 42 at a first angle of incidence. The second semiconductor laser diode 38 is provided for illuminating the diffraction grating 32 with the second beam 44 at a second angle of incidence, as shown with the keyed dashed lines indicating first and second beams 42 and 44 respectively. An output array 46 is provided for receiving the first and second output beams 42" and 44" at the output 46 for coupling the zeroth-order reflections from the diffraction grating.

With the one-dimensional array 40 of lasers in the system 30, every laser receives feedback at a different wavelength resulting from different angles, θ' and θ", between the direction of laser output from the array and the mirror. Thus, the mirror does not rotate or translate. Instead, the angle θ changes for every diode because of its position as shown in FIG. 2. This construction provides for illumination of the grating 32 surface at different incident angles for multiple lasers with the fixed mirror 34 allowing for multiple standing waves within the single resonating cavity. It will be appreciated that care must be taken to avoid mode hopping as one progresses down the array. This can be controlled by carefully orienting the array or placing compensating optics in the path of some of the emitters. The linear array of lasers 40 which include lasers 36 and 38, and which further anticipate a continuum of lasers along the linear array to provide multiple such lasers positioned to facilitate incident angles θ, allows for a continuous change of wavelength without mode hopping, and also without the need for moving parts to achieve the differing angles of incidence θ provided by the linear array of lasers 40 as discussed herein. To provide wavelength changes free from mode hopping, the resonator 30 facilitates a number of nodal planes in the standing wave of the resonator kept unchanged as new wavelengths associated with the individual lasers of the laser array 40 are provided at differing angles of incidence θ to provide additional wavelengths for the resonant cavity of the system 30. Accordingly, the positioning of the lasers in the laser array achieves the angles of incidence for the wavelengths generated in the system 30 without the need for mechanical manipulation of the mirror 34 which remains fixed with the different spatial positions being provided by the lasers of the linear laser array 40. The diffraction grating 32 is illuminated at differing angles of incidence to diffract corresponding different wavelengths (λs) upon the mirror 34 for the first-order reflection as discussed above. The multiple λs reflected from the mirror 34 and diffracted with the grating 32 go back to the corresponding first, second, third, etc. lasers of the array 40 such that the grating 32 causes dispersion in the corresponding beams such that light coming back into the laser array 40 is directed to the corresponding particular lasers, e.g., 36, 38. The relative spatial orientation between the lasers 36, 38 of the array 40 and the mirror 34 at angles θ' and θ" are provided with angles sufficient to avoid cross-talk in the different wavelength feedback paths with the distance provided between the laser emitters and the cavity.

The system 50 of FIG. 3 provides the linear array of lasers as a two- dimensional array of lasers 60, which when operated in conjunction with the cross-dispersed Littrow grating 54, facilitate two-dimensional orientation of the λ paths being arranged in the resonator. Herein, laser diodes 56 and 58 illuminate the grating 52 as indicated by paths 62 and 64 respectively for generating differing λs providing a resonating cavity with the grating 54 directing the first-order reflections 62' and 64' back to the grating 52 and directed to the corresponding laser of the two-dimensional array of lasers 60. Accordingly, the corresponding λ associated with indicated paths 62" and 64" are provided as the laser output zeroth-order from output 66 providing the laser beam output according to a two-dimensional array.

FIG. 3 is a schematic diagram of the modified design of system 50 provided to wavelength lock a two-dimensional array of semiconductor lasers. In the case of locking the wavelengths of a two-dimensional array of emitters, the cavity design for a two-dimensional diode laser array is further modified by replacing the mirror by a grating 54 in a Littrow cross-dispersed configuration. Light from a two-dimensional array of laser emitters is collimated and impinges on the first grating in a grazing incidence geometry. With the mirror replaced with the second, cross-dispersing grating 54 in the Littrow configuration, the grating 54 then reflects back light of the desired wavelength in a direction which is opposite to that of the incident light. The grating 54 is often designed for use in the first-order of interference. In the system 50, the wavelength feedback to the two-dimensional laser diode array is a function of position in two dimensions allowing one to lock the wavelength of every position element in the array to a slightly different wavelength. Locking a two- dimensional array requires two-dimensional feedback. By using light of different wavelengths fed back from a two-dimensional array of lasers 60 to each position on the two-dimensional emitter array, each laser operates at a different wavelength. In this case, care must be taken to space the emitters, e.g., diodes 56 and 58, differently in the x and y directions on the emitter or to use grating with different rulings so that no wavelength is repeated in the array. Alternatively, use of a specialized echelle grating in a Littrow mount configuration, discussed below, may also be used to achieve the desired two-dimensional wavelength feedback. Because the feedback is double passed over the first grating, the system achieves a very high resolution so that each laser's bandwidth is limited by uncertainty broadening due to the fast pulse width rather than the bandwidth of the feedback. Although DFB lasers have narrow line operation, they are subject to unacceptable wavelength draft, such that pulsed operation of DFB lasers is limited by uncertainty broadening. The first grating 52 directs the first-order reflection of each laser, while still collimated, to the second grating 54 in Littrow configuration that is oriented to cross-disperse the light. Since the Littrow configuration returns nearly all light in the direction from whence it came, the light (still collimated) returns to the first grating where the first-order reflection passes directly back to the laser array. However, the wavelength of the feedback is position dependent, and therefore, each laser in the two-dimensional array receives feedback at a different wavelength. Feedback causes a diode laser to lock to the wavelength of the feedback so that each laser in the array lases at a discrete, well-controlled wavelength. Light coupling out of the laser occurs by taking the zeroth-order reflection from the first grating. The lasing bandwidth of each line is quite narrow since the light is transmitted to the first grating in grazing incidence two times. A relatively small array, e.g., 33 x 33 emitters, provides 1000 independent wavelengths for multiplexing on a single fiber.

While the preferred embodiment described herein contemplates the diffracted feedback wavelengths to be first-order reflections and the output reflections to be zeroth-order, they could be different orders of reflection. Preferably, the feedback wavelength is always at a greater order of reflection than the output reflections. It will be appreciated by those skilled in the art that modifications to the foregoing preferred embodiments may be made in various aspects. The present invention is set forth with particularity in the appended claims. It is deemed that the spirit and scope of the invention encompasses such modifications and alterations to the preferred embodiment as would be apparent to one of ordinary skill in the art and familiar with the teachings of the present application.

Claims

What is claimed is:

1. An external cavity diode laser system for plural wavelength locked beams with a single resonant cavity, the system comprising: a diffraction grating; a first semiconductor diode laser for illuminating said diffraction grating with a first beam at a first angle of incidence; a second semiconductor diode laser for illuminating said diffraction grating with a second beam at a second angle of incidence; a reflector opposite said diffraction grating positioned to reflect the first beam and the second beam deflected from said diffraction grating back to said first laser and said second laser respectively; and an output for coupling the first beam and the second beam from said diffraction grating.

2. A system as recited in claim 1, wherein the first beam and the second beam are deflected from said diffraction grating to provide a first-order reflection from said diffraction grating.

3. A system as recited in claim 1, wherein said reflector comprises a mirror.

4. A system as recited in claim 1, wherein said reflector, said first laser, and said second laser are positioned relative to said diffraction grating for causing the first beam to resonate at a first wavelength and the second beam to resonate at a second wavelength different from and locked with the first wavelength.

5. A system as recited in claim 1, comprising a linear array of semiconductor diode lasers comprising a third semiconductor diode laser for illuminating said diffraction grating with a third beam at a third angle of incidence for resonating the third beam at a third wavelength locked relative to the first wavelength and the second wavelength.

6. A system as recited in claim 5, wherein said linear array of semiconductor diode lasers comprises a multiplicity of semiconductor diode lasers.

7. A system as recited in claim 1, wherein said output couples zeroth- order reflections from said diffraction grating.

8. A system as recited in claim 1, wherein said reflector comprises a second diffraction grating.

9. A system as recited in claim 8, wherein said second diffraction grating comprises a Littrow mount positioning for said second diffraction grating.

10. A system as recited in claim 8, wherein said second diffraction grating comprises a cross-dispersed diffraction grating.

11. A system as recited in claim 8, comprising a two-dimensional array of lasers.

12. A wavelength locking method for plural diode laser beams in a single resonant cavity using a diffraction grating in an external cavity, the method comprising: illuminating the diffraction grating with a first beam from a first semiconductor diode laser at a first angle of incidence; illuminating the diffraction grating with a second beam from a second semiconductor diode laser at a second angle of incidence; positioning a reflector opposite the diffraction grating to reflect the first beam and the second beam deflected from the diffraction grating back to the first laser and the second laser respectively; and coupling the first beam and the second beam from the diffraction grating for wavelengths locked relative to the first laser and the second laser beams.

13. A method as recited in claim 12, comprising positioning the reflector, the first laser, and the second laser relative to said diffraction grating for causing the first beam to resonate at a first wavelength and the second beam to resonate at a second wavelength different from and locked with the first wavelength.

14. A method as recited in claim 12, comprising illuminating the diffraction grating with a third beam from a third semiconductor diode laser at a third angle of incidence for resonating the third beam at a third wavelength locked relative to the first wavelength and the second wavelength, wherein the first, second, and third diode lasers are provided in a linear array of semiconductor diode lasers.

15. A method as recited in claim 14, comprising providing a multiplicity of semiconductor diode lasers in the linear array of semiconductor diode lasers.

16. A method as recited in claim 12, wherein said coupling step couples zeroth-order reflections from said diffraction grating.

17. An external cavity diode laser system for plural wavelength locked beams with a single resonant cavity, the system comprising: first means for illuminating a diffraction grating with a first beam from a first semiconductor diode laser at a first angle of incidence; second means for illuminating the diffraction grating with a second beam from a second semiconductor diode laser at a second angle of incidence; means for positioning a reflector opposite the diffraction grating to reflect the first beam and the second beam deflected from the diffraction grating back to the first laser and the second laser respectively; and means for coupling the first beam and the second beam from the diffraction grating for wavelengths locked relative to the first laser and the second laser beams.

18. A system as recited in claim 17, wherein said first and second means for illuminating comprise a linear array of semiconductor diode lasers comprising a multiplicity of semiconductor diode lasers.

19. A system as recited in claim 17, wherein said means for positioning the reflector comprises a second diffraction grating.

20. A system as recited in claim 19, wherein the second diffraction grating comprises a cross-dispersed diffraction grating positioned with a Littrow mount.