SPECTRALLY AND SPATIALLY MISMATCHED SEEDING OF A MULTIMODE VCSEL FOR
MODULATION BANDWIDTH ENHANCEMENT
This applications claims the benefit of and priority to United States Patent application number 61/049,868, filed May 2, 2008.
The present disclosure relates to the use of multimode vertical-cavity surface- emitting lasers (VCSELs) in optical systems and, more particularly to optical injection locking (OIL) techniques for improving the direct modulation performance of multimode VCSELs. Examples of specific applications for the aforementioned laser sources include, but are not limited to, high-speed short-reach fiber-optic networks and radio-over-fiber systems, among others.
The present inventors have recognized that multimode VCSELs can be attractive components of a laser source because they can be manufactured cost-effectively with larger tolerance and yield than single mode VCSELs. The present inventors have also recognized, however, that the multimode nature of the VCSEL can introduce modal competition noise and modal dispersion in the laser source - factors that can prevent these devices from being used for applications that demand high modulation speeds and/or long transmission distance.
According to the present disclosure, significantly enhanced bandwidth and transmission distance can be achieved using optical injection locking (OIL) as a technique to improve the direct modulation performance of multimode VCSELs. As a result, laser sources incorporating optically injection locked multimode VCSELs can be configured as high-speed, low-cost optical transmitters and can function as important components for next generation 100-Gb/s Ethernet and local area networks (LANs). For example, the present disclosure contemplates that optical injection locking can be used with a multimode VCSEL having a free-running 1550 nm multimode VCSEL having a 3dB- bandwidth of 3 GHz to yield a 54 GHz resonance frequency and 38 GHz 3-dB bandwidth. Moreover, the techniques are readily applicable to other wavelengths such as 850 nm or 980 nm.
The aforementioned performance is made possible by leveraging the unique properties of the multimode VCSEL, which typically has spatially and spectrally well- separated modes. As is described in further detail below, this separation facilitates efficient injection locking preferentially to, but not limited to, the fundamental transverse mode. It is contemplated that the techniques described herein are suitable for a variety of different modulation formats, such as amplitude modulation, phase modulation, or frequency modulation, on either the slave laser or the master laser.
According to one embodiment of the present disclosure, a method of operating a laser source comprising a single mode master laser and a multimode VCSEL slave laser is provided. According to the method, the spatial and spectral coupling of the single mode optical output of the master laser and a target mode of the multimode optical resonator of the VCSEL slave laser are controlled to progress from a relatively mode-matched spatial and spectral coupling to a relatively mismatched spatial and spectral coupling to facilitate optically injected mode locking in the laser source. For the purposes of describing and defining the present invention, it is also noted that the present inventors have recognized that multimode VCSELs typically emit multiple transverse modes, which are at different wavelengths and have different spatial power distributions. At any given bias current or modulation conditions, these modes can exist simultaneously. Using the injection locking techniques described herein, any of these modes can be selected by matching the optical intensity profile and spectra of the master laser to that given mode. The given mode is herein referred to as the target mode.
According to another embodiment of the present disclosure, a laser source is provided comprising a single mode master laser and a multimode VCSEL slave laser. The single mode master laser comprises a wavelength tuning element and a spatial tuning element that facilitate optically injected mode locking in the laser source. The injection locking techniques described herein use the single mode master laser to optically lock the multimode VCSEL slave laser, which can be directly modulated. The resulting laser source exhibits increased laser resonance frequency and bandwidth, reduced non-linear distortions, and reduced frequency chirp.
The following detailed description of specific embodiments of the present disclosure can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which:
Fig. 1 is a schematic illustration of the spatial progression of the optical output of a single mode master laser relative to the fundamental and first order transverse modes of the multimode optical resonator of a VCSEL slave laser; and
Fig. 2 is a schematic illustration of a laser source comprising a single mode master laser and a multimode VCSEL slave laser
As is illustrated in Figs. 1 and 2, laser sources 100 according to the present disclosure comprise a single mode master laser 10 and a multimode VCSEL slave laser 20. In operation, the single mode optical output 12 of the master laser 10 is spatially and spectrally coupled to the optical resonator 22 of the multimode VCSEL slave laser 20 at an injection ratio PMASTER/PSLAVE that is sufficient to stably injection lock the optical resonator 22 and generate a secondary output beam 25. According to the optical injection locking (OIL) techniques described herein, the spatial and spectral coupling of the single mode optical output 12 of the master laser 10 and a target mode of the multimode optical resonator 22 are controlled to progress from a relatively mode-matched spatial and spectral coupling to a relatively mismatched spatial and spectral coupling. This progression facilitates optically injection locking in the laser source 100 and is described in further detail below.
Fig. 1 illustrates one example of the manner in which the laser source 100 can be operated to progress from a relatively mode-matched spatial coupling, where the single mode optical output 12 of the master laser 10 is spatially aligned with the target mode of the multimode optical resonator 22, to a relatively mismatched spatial coupling, where the single mode optical output 12 of the master laser 10 is primarily coupled to a peripheral spatial portion of the target mode of the multimode optical resonator 22.
The spatial coupling can be controlled by adjusting the beam spot position of the single mode optical output 12 relative to the input aperture of the multimode optical resonator 22. For example, and not by way of limitation, the beam spot position can be
adjusted by translating the optical output 12 of the master laser 10 along a dimension that is approximately parallel to the input aperture of the multimode optical resonator 22. Alternatively, the beam spot position can be adjusted by steering the output beam, i.e., by altering the angle of incidence of the optical output 12 of the master laser 10 relative to the input aperture of the multimode optical resonator 22. The aforementioned translation and beam steering are illustrated schematically in Fig. 1 by directional arrows 15.
Spatial coupling can be further facilitated by providing the single mode optical output 12 of the master laser 10 with an optical element that is configured to restrict the cross section of the optical output 12 at the input aperture of the multimode optical resonator 22 to a fraction of the cross section of the input aperture. For example, and not by way of limitation, a lensed or cleaved optical fiber can be utilized to restrict the cross section of the optical output 12 to less than about 'Λ of the cross section of the input aperture of the multimode optical resonator 22. In many embodiments, the cross section of the optical output of the master laser will be between approximately 2 μm and approximately 10 μm and the cross section of the input aperture of the multimode optical resonator will be between approximately 7 μm and approximately 50 μm.
The aforementioned progression in spectral coupling can be illustrated with reference to Table 1. Table 1 presents data that illustrates progression from a relatively mode-matched spectral coupling to a relatively mismatched spectral coupling. Specifically, under mode-matched spatial coupling the single mode optical output 12 of the master laser 10 starts with a center wavelength λ
MASTER that is greater than or relatively close to the fundamental transverse mode center wavelength λsuwε of the multimode VCSEL slave laser, e.g., λ
MASTER-λs
LAvε
= 0.48 nm. The single mode optical output 12 subsequently progresses to a relatively mismatched spectral coupling, where the single mode optical output 12 of the master laser 10 comprises a center wavelength λ
MASTER that is significantly less than the fundamental transverse mode center wavelength λsLAVE of the multimode VCSEL slave laser, e.g., λMASTER-λsLAvε = -0.366 nm. Intermediate spectral coupling values are also illustrated in the table, any one of which could be viewed as a suitable condition for relatively mismatched spatial coupling:
For the purposes of describing and defining the present invention, it is noted that the injection ratio PMASTER/PSLAVE is measured as a ratio of optical power estimated to be incident on the VCSEL versus the output power of the free running VCSEL. Also, it is noted that the value λMASTER-λsLAVE is measured as the wavelength difference of the master laser and the free running VCSEL.
It is noted that the spectral progression illustrated in Table 1 can be characterized as a progression from a relatively mode-matched state to a relatively mismatched state because, in progressing from λMASTER-λsLAVE = 0.48 nm to λMASTER-λSLAVE = -0.366 nm, the wavelength of the master laser will necessarily pass through λMASTER-λsLAVE = 0 nm, the ideal mode-matched spectral state, to λMASTER-λsuwε = -0.366 nm, a relatively mismatched spectral state. In executing the spectral progression of the present disclosure, it is noted that relatively mode-matched spectral states need not include the ideal mode-
matched spectral state. Rather, the master laser must merely progress from a spectral state that is mode-matched enough to facilitate mode locking upon progression to the relatively mismatched spectral state. Specific wavelength values or relationships for these two types of states will vary depending upon the respective configurations of the master and slave lasers.
The aforementioned spatial and spectral progressions for the case where the first order transverse modes illustrated in Fig. 1 is the target mode, would be analogous to those described above with respect to the fundamental transverse mode, with the exception that the mode-matched spectral and spatial states would be shifted to match the first order transverse mode, as is illustrated in the Table 2:
The data in Tables 1 and 2 represent the progression of a master/slave configuration from a mode-matched spatial state to a mismatched spatial state, as is illustrated in Fig. 1.
Tables 1 and 2 also illustrate injection power ratios and resonance frequencies of the injection locked laser sources as the master laser progresses towards the relatively mismatched spectral state.
By utilizing a power controller 50, a wavelength-tunable laser controller 60, and an output beam positioner 70 to optimize the injection power ratio and the progression of the spatial and spectral states towards a mismatched state, a regime with efficient and stable locking can be found. Injection power ratios between approximately -5 dB and approximately 30 dB are likely to be suitable but a variety of injection power ratios are contemplated. For example, the injection ratio PMASTER/PSLAVE and the spatial and spectral coupling can be controlled to achieve a resonance frequency exceeding approximately 50 GHz and a 3 dB bandwidth exceeding approximately 30 GHz. A single mode to multimode transformer can be used to increase the injection ratio PMASTER/PSLAVE and may, for example, comprise a 3-port fused fiber coupler comprising a single-mode fiber port as a master laser input port, a hybrid multimode/single-mode fiber port for in and out coupling the multimode VCSEL slave laser, and a multimode or single mode fiber output port.
The significant improvements in resonance frequency over a range of spectral coupling conditions show that a variety of spectral coupling conditions could be viewed as a suitable condition for the aforementioned relatively mismatched spatial coupling. To avoid performance degradation, it is desirable to have the laser resonance frequency greatly exceed the highest RF frequency of use. Spectral coupling can be controlled by providing a master laser that is a wavelength tunable laser and by tuning the center wavelength λMASTER of the single mode optical output 12 of the master laser 10 to a suitable value, typically by a fraction of a nanometer. The single mode master laser 10 may, for example, comprise a DBR laser, a DFB laser, a Fabry Perot laser, a VCSEL, or a fiber laser that is either structurally independent of or structurally integrated with the slave laser 20 and can be configured to stably injection lock the multimode VCSEL slave laser 20 via top face, bottom face, or external cavity injection locking.
As is evident from the data presented in Tables 1 and 2, the center wavelength λMASTER can initially be greater than or less than the target mode center wavelength λsLAvε of the multimode VCSEL slave laser as it progresses towards the aforementioned mismatched spectral coupling. In many cases, it is contemplated that the spectral coupling of the single mode optical output 12 of the master laser 10 should be controlled such that the relatively mode-matched spectral coupling and the relatively mismatched spectral coupling are separated by a wavelength spacing that is merely a fraction of the mode spacing between the fundamental mode and the first order transverse mode of the multimode optical resonator. For example, it is contemplated that the spectral coupling of the single mode optical output 12 of the master laser 10 can be controlled such that the relatively mode-matched spectral coupling and the relatively mismatched spectral coupling are separated by a wavelength spacing of between approximately 0.1 nm and approximately 1.5 nm. In some cases, it may merely be sufficient to ensure that the spectral coupling of the single mode optical output 12 is controlled such that the center wavelength XMASTER falls between the fundamental mode and a first order transverse mode of the slave resonator 22.
The multimode VCSEL slave laser 20 can be biased to force multimode operation and to enhance modulation bandwidth. For example, it would not be unusual for the first order transverse modes to fall at a wavelength that is approximately 1 nm to approximately 2 nm shorter than the fundamental mode. In which case, the fundamental mode locking range could range from about 1.5 nm shorter to about 3 nm longer than the center wavelength of the fundamental mode.
Although not required, the laser source 100 should further comprise a polarization controller 30 because a biased multimode VCSEL slave laser will often emit fundamental and first-order transverse modes that comprise at least two different polarization modes. The polarization controller 30 can be used to match the respective polarizations of the single mode optical output 12 and the target mode of the multimode VCSEL slave laser 20. Further, the single mode optical output 12 of the master laser 10 can be optically coupled to the optical resonator 22 of the multimode VCSEL slave laser 20 through an
optical circulator 40 to prevent optical feedback to the master laser 10. It is also contemplated that the single mode optical output 12 of the master laser 10 can be optically coupled to optical resonators of an array of multimode VCSEL slave lasers via, for example, a 3 dB optical splitter, a beam splitter, or a polarizing beam displacer. Finally, it is contemplated that the single mode optical output 12 of the master laser 10 can be optically coupled to the optical resonator of the multimode VCSEL slave laser through a coupling fiber that is mode matched with a fundamental or higher order transverse optical mode of the multimode VCSEL slave laser.
It is noted that recitations herein of a component of the present disclosure being "configured" in a particular way, to embody a particular property, or to function in a particular manner, are structural recitations, as opposed to recitations of intended use. More specifically, the references herein to the manner in which a component is "configured" denotes an existing physical condition of the component and, as such, is to be taken as a definite recitation of the structural characteristics of the component. For the purposes of describing and defining the present invention it is noted that the term "substantially" is utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. The term "substantially" is also utilized herein to represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue.
It is noted that terms like "preferably," "commonly," and "typically," when utilized herein, are not utilized to limit the scope of the claimed invention or to imply that certain features are critical, essential, or even important to the structure or function of the claimed invention. Rather, these terms are merely intended to identify particular aspects of an embodiment of the present disclosure or to emphasize alternative or additional features that may or may not be utilized in a particular embodiment of the present disclosure.
For the purposes of describing and defining the present invention it is noted that the term "approximately" is utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other
representation. The term "approximately" is also utilized herein to represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue.
Having described the subject matter of the present disclosure in detail and by reference to specific embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims. More specifically, although some aspects of the present disclosure are identified herein as preferred or particularly advantageous, it is contemplated that the present disclosure is not necessarily limited to these aspects. It is noted that one or more of the following claims utilize the term "wherein" as a transitional phrase. For the purposes of defining the present invention, it is noted that this term is introduced in the claims as an open-ended transitional phrase.