Patent Publication Number: US-2003235230-A1

Title: Method and apparatus for mode-locked vertical cavity laser with optimized spectral bandwidth

Description:
RELATED APPLICATIONS  
     [0001] This application is a continuation in part of U.S. Application Ser. No. 09/817,362, filed Mar. 20, 2001. This application also claims the benefit of U.S. Provisional Applications 60/263,060, filed Jan. 19, 2001; and 60/xxx,xxx, filed Jul. 6, 2001, U.S. Express Mail No. ET161056037US, Attorney Docket No. Siros-031. 
    
    
     
       BACKGROUND OF THE INVENTION  
       [0002] I. Field  
       [0003] The present disclosure relates to Vertical Cavity Surface Emission Lasers (VCSELs).  
       [0004] II. Background  
       [0005] Fiber optical networks are becoming increasingly faster and more complex. Key to this expansion are technologies such as Vertical Cavity Surface Emission Lasers (VCSELs) because of their cost.  
       [0006] As is known by those skilled in the art, VCSEL are currently favored over competing technologies such as edge-emitting lasers because VCSELs may be tested while still in wafer form, while edge-emitting laser typically must be dice-cut prior to testing. This is because edge-emitting lasers must be cleaved in order to emit light, while VCSELs do not require cleaving and may emit light while still in wafer form.  
       [0007] However, one challenge to the implementation of VCSELs in modern systems is that current VCSELs may operate well at some wavelengths but not at others. Additionally, some VCSELs may display frequency instability or insufficient power output. Hence, most VCSEL devices are typically employed in applications where exact frequency output is unimportant.  
       [0008] As is appreciated by those skilled in the art, the ultimate limit on emission bandwidth is given by the gain bandwidth of the quantum well region. However, the target response of a given device may often be less than the total emission bandwidth possible for the device. For example, a 16-channel system with 25 GHz spacing requires a total spectral bandwidth of 375 GHz. If the spectral bandwidth of the quantum well structure is greater than 375 GHz, then there is a potential for extraneous channels to be generated outside of the desired spectral band. This is undesirable because of the resulting waste of energy.  
       [0009] Hence, there is a need for a mutlichannel light source that does not suffer from the deficiencies of the prior art.  
       SUMMARY  
       [0010] A multi-frequency light source is disclosed. In one aspect, a multi-frequency light source may comprise a gain region defined by a first and second mirror. The gain region may have a corresponding resonant mode. The light source may also have an external cavity defined by a third mirror and the second mirror. The external cavity has plurality of resonant modes, including a plurality of contiguous desired modes of operation. The second mirror may be formed such that the multi-frequency light source operates at the desired modes of the external cavity. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0011] The features, objects, and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein:  
     [0012]FIG. 1 is a conceptual diagram of one aspect of a disclosed system;  
     [0013]FIG. 2 is a more detailed conceptual diagram of one aspect of a disclosed system;  
     [0014]FIG. 3 is a plot of the resonant modes of one aspect of a disclosed system;  
     [0015]FIG. 4 is a plot of the resonant bandwidth of one aspect of a disclosed system; and  
     [0016]FIG. 5 is a another plot of the resonant modes of one aspect of a disclosed system. 
    
    
     DETAILED DESCRIPTION  
     [0017] Persons of ordinary skill in the art will realize that the following description of the present invention is illustrative only and not in any way limiting. Other embodiments of the invention will readily suggest themselves to such skilled persons having the benefit of this disclosure.  
     [0018] The following references are hereby incorporated by reference into the detailed description of the preferred embodiments, and also as disclosing alternative embodiments of elements or features of the preferred embodiment not otherwise set forth in detail above or below or in the drawings. A single one or a combination of two or more of these references may be consulted to obtain a variation of the preferred embodiment described above. In this regard, further patent, patent application and non-patent references, and discussion thereof, cited in the background and/or elsewhere herein are also incorporated by reference into the detailed description with the same effect as just described with respect to the following references:  
     [0019] U.S. Pat. Nos. 5,347,525, 5,526,155, 6,141,127, and 5,631,758;  
     [0020] Wilmsen, Temkin and Coldren, et al., “Vertical Cell Surface Emitting Lasers, 2nd edition;  
     [0021] Ulrich Fiedler and Karl Ebeling, “Design of VCSELs for Feedback Insensitive Data Transmission and External Cavity Active Mode-Locking”, IEEE JSTQE, Vol. 1, No. 2 (June 1995); and  
     [0022] J. Boucart, et al., 1-mW CW-RT Monolithic VCSEL at 1.55 mm, IEEE Photonics Technology Letters, Vol. 11, No. 6 (June 1999).  
     [0023]FIG. 1 is a conceptual diagram of a multichannel light source and illustrates a three-mirror composite-cavity VCSEL configured in accordance with the teachings of this disclosure. The light source includes epitiaxially-grown mirrors M 1  and M 2 , and an external mirror M 3 . In operation, mirror M 3  controls frequency spacing between mode-locked modes by way of its distance from M 2  and M 3  (representing a cavity length L 2 ), and provides output coupling of the laser energy. The combination of these mirrors defines two cavities: the VCSEL resonant cavity  2 , or gain cavity  2 , defined by M 1  and M 2 ; and an external cavity  4  defined by M 2  and M 3 .  
     [0024]FIG. 2 is a more detailed conceptual diagram of one aspect of a disclosed multifrequency light source  100 . The light source  100  may include a VCSEL  101  having a substrate  102  for reflecting light at normal incidence. The substrate  102  may be formed from materials known in the art such as Gas or InP depending on the desired wavelength.  
     [0025] On top of the substrate  102  a mirror M 1  is formed. The layers of M 1  may be formed epitaxially using techniques known in the art. If the substrate  102  comprises GaAs, then the layers of M 1  may be formed from alternating layers of GaAs/InGaAs for use in the wavelength range of 780-980 nm. Alternatively, if the substrate  102  comprises InP, the layers of M 1  may formed of alternating layers of InGaAlAs/InP for use in the wavelength range of 1300-1700 nm.  
     [0026] An active layer  104  for amplifying light is then grown on M 1 . The active layer  104  may comprise a quantum well active layer fashioned from the same materials as M 1 . The active layer  104  will have a gain response and a nominal peak frequency associated therewith. In one aspect of a disclosed light source, the active layer  104  may have a nominal peak frequency of 1550 nm. The nominal peak frequency are typically functions of variables such as current or temperature.  
     [0027] A mirror M 2  may then be grown on the active layer  104  using techniques similar to M 1 . The active layer  104 , combined with mirror layers  102  and M 3  comprise a resonant cavity for which can be associated an effective cavity length L 1 .  
     [0028] The light source  100  may further include a mirror M 3  disposed a distance L 2  from the upper surface of M 2 .  
     [0029] A multifrequency light source  100  is thus formed including a VCSEL  101  and an external mirror M 3  wherein several alternative designs and variations may be possible. The light source  100  may be described in terms of the distance L 1  between mirrors M 1  and M 2  forming a gain cavity and the distance L 2  between mirrors M 2  and M 3  forming an external cavity.  
     [0030] In general, the cavity length of the external cavity may be greatly extended compared with a conventional VCSEL device. The external cavity may be, e.g., between a few hundred microns and several millimeters, and is particularly preferred around 2-3 mm in physical length for a mode-spacing of 50 GHz. For example, at 50 GHz and for a refractive index n=1 (such as for an air or inert gas filled cavity), then the cavity will have a physical length L 2  of about 3 mm, which provides a 3 mm optical path length corresponding to 50 GHz. The actual cavity length to achieve 50 GHz may also depend on the reflective indices of the media between M 2  and M 3 . For example, for a cavity material such as glass, e.g., n=1.5, then the physical length will be around 2 mm to provide the optical path length of 2 mm×1.5=3 mm, again corresponding to a 50 GHz mode spacing.  
     [0031] The distance L 2  and thus the cavity length may be increased to reduce the mode-spacing. For example, by doubling the cavity length, e.g., to 4-6 mm, the mode-spacing may be reduced to 25 GHz, or by again doubling the cavity length, e.g., to 8-12 mm, the mode-spacing may be reduced to 12.5 GHz. The mode-spacing may be increased, if desired, by alternatively reducing the cavity length, e.g., by reducing the cavity length to half, e.g., 1-1.5 mm to increase the mode-spacing to 100 GHz. Generally, the mode-spacing may be advantageously selected by adjusting the cavity to a corresponding cavity length. The device of the preferred embodiment may utilize other means for reducing the mode-spacing as understood by those skilled in the art.  
     [0032] This extension of cavity length from that of a conventional VCSEL is permitted by the removal or partial removal of a mirrored reflector surface of the mirror M 2  and inclusion of mirror M 3 . The light source  100  and in particular the mirror M 3  may be formed as disclosed in co-pending application No. 09/817,362, filed Mar. 20, 2001, and assigned to the same assignee of the present application, and incorporated by reference as though set forth fully herein.  
     [0033] The extension of the external cavity out to 1.5-15 mm permits a 10-100 GHz mode spacing, since the cavity will support a number of modes having a spacing that depends on the inverse of the cavity length (i.e., c/2 nL, where c is the speed of light in vacuum, n is the refractive index of the cavity material and L is the cavity length). The VCSEL with external cavity device for providing multiple channel signal output according to a preferred embodiment herein is preferably configured for use in the telecom band around 1550 nm, and alternatively with the telecom short distance band around 1300 nm or the very short range 850 nm band. In the 1550 nm band, 100, 50 and 12.5 GHz cavities are of particular interest as they correspond to standard DWDM channel spacings.  
     [0034] The monolithic portion of the light source  100  may be around 15 microns tall when formed on a substrate 100-700 μm thick and preferably comprises a gain medium of InGaAsP or InGaAs and InGaAlAs or In GaAsP or AlGaAs mirrors (or mirrors formed of other materials according to desired wavelengths as taught, e.g., in Wilmsen, Temkin and Coldren, et al., “Vertical Cavity Surface Emitting Lasers, 2nd edition, Chapter 8).  
     [0035] The light source  100  may be formed in a variety of manners. For example, the second mode spacing cavity may be formed by a solid lens of either conventional or gradient index design, and may be formed of glass. When a gradient index lens is used, the index of refraction of the material filling the cavity varies (e.g., decreases) with distance from the center optical axis of the resonant cavity. Such GRIN lens provides efficient collection of the divergent light emitted from the laser cavity. In an embodiment using a GRIN lens, the mirrored surface of mirror M 3  may be curved or flat, depending on design considerations.  
     [0036] The mirror M 3  may have one or more coatings on its remote surface such that it efficiently reflects incident light emitted from the VCSEL  101  as a resonator reflector, preferably around 1550 nm for the telecom band. The mirror M 3  is preferably formed of alternating high and low refractive index materials to build up a high reflectivity, such as alternating quarter-wavelength layers of TiO2/SiO2 or other such materials known to those skilled in the art.  
     [0037] The radius of curvature of the lens may be around the length the second cavity. Emitted radiation from the VCSEL  101  will diverge outward from the gain region substantially be reflected directly back into the gain region when the radius of curvature is approximately the cavity length, or around 2-3 mm for a 50 GHz mode-spacing device.  
     [0038] The two cavities of the light source  100  will each have corresponding resonant modes associated therewith, as illustrated in FIG. 3. The resonant modes for the external cavity defined by the distance L 2  are shown as plot  300 , and corresponding resonant mode plot for the gain cavity defined by the distance L 1  is shown as plot  310 .  
     [0039] In operation, the cavities provide one or more resonant nodes at optical frequencies for which the roundtrip gain exceeds the loss. For a longer cavity such as the external cavity, the resonant nodes form a comb of frequencies having a separation inversely proportional to the cavity length. For example, for a cavity optical length of 3 mm, the optical spacing of the modes is approximately 50 GHz. The light amplifying active layer will typically have a gain bandwidth of 2-4 THz (2000-4000 GHz). Thus, many such nodes will fit within the gain bandwidth of the gain material.  
     [0040] However, the gain cavity of the VCSEL gain cavity typically has a micron-scale optical length and thus a much greater modal spacing, typically in the multi-THz range. Since L 2 &gt;&gt;L 1 , many more resonant modes will occur in the external cavity in a given frequency spectrum than will occur in the gain cavity. In fact in the typical instance, there may be only one resonant mode in the first cavity which falls in the corresponding gain bandwidth of the laser, as is illustrated in FIG. 3.  
     [0041] Thus, typically only one resonance will exist in the gain bandwidth. The breadth of this resonance depends on the values of M 2  and M 3  and may range from a few GHz to 1 THz.  
     [0042] When the two cavities defined by (M 1  and M 2 ) and (M 2  and M 3 ) are put together, they must jointly satisfy roundtrip phase boundary conditions for laser operation. If the modes of the second cavity do not overlap with at least one of the modes of the first cavity, then laser emission will not be achieved.  
     [0043] Thus, when combining the fine comb frequencies of an external cavity with the single resonance of a VCSEL gain cavity, lasing may be limited to cavity resonances which lie within the resonant bandwidth of the VCSEL gain cavity. The width of the resonance of the VCSEL cavity may be varied by varying M 1  and M 2 , such that the gain cavity resonance can span multiple external resonances.  
     [0044] Two typical specifications for VCSEL-based systems are channel frequency spacing and number of channels. The product of these two represents the emission bandwidth and is therefore an essential requirement for any VCSEL device.  
     [0045] The response of the device thus depends on the relative reflectivity of the mirrors, the gain response and bandwidth of the amplifying region, and the relationship between the resonances of the gain regions of both cavities.  
     [0046] In one aspect of a disclosed multi-frequency light source, the spectral bandwidth of the light source may be controlled by varying the reflectivity of M 2 . The reflectivity of M 2  may be controlled by altering the number of layer pairs used to form the mirror. As the reflectivity of M 2  is decreased, the spectral bandwidth will increase.  
     [0047]FIG. 4 is plot showing the reflectivity of M 2  for two disclosed aspects. In one aspect, M 1  comprises a 35 layer-pair mirror and M 2  comprises a 7 layer-pair mirror; and in a second aspect, M 1  comprises a 35 layer-pair mirror and M 2  comprises a 23 layer pair mirror. As will be appreciated from FIG. 4, the 23 layer-pair structure has a resonance width which is more narrow than the 7 layer-pair stack. Since the response of the 23 layer-pair structure is an order of magnitude more narrow than the 7 layer-pair structure, coupling the 7 layer-pair structure with an external cavity will result in a much greater spectral range.  
     [0048]FIG. 5 is a conceptual plot showing how the reflectivity of M 1  may be adjusted to achieve mode selectivity. FIG. 5 includes the resonant modes of an external cavity  400  plotted above the resonant mode of a VCSEL gain cavity  410  along a common frequency axis. FIG. 5 further shows how varying the reflectivity of the gain cavity may result in different responses M 2 ′, M 2 ′, and M 2 ′″. By analogy to the electrical arts, by varying the Q of the gain cavity, the resonant bandwidth of the gain cavity may be selected advantageously. As the reflectivity of the mirror is reduced, the resonance flattens out, as in a lower-Q circuit.  
     [0049] As will be appreciated from FIG. 5, by varying the reflectivity of M 2 , the spectral bandwidth of the gain cavity may be chosen so as to have a predetermined response shape. By varying the response shape of the mirror, the response of the gain cavity may be shaped so as to overlap or intersect one or more of the resonant modes of the external cavity. The reflectivity of M 2  may be chosen such that the resonant mode of the gain cavity is substantially equal in frequency to one or more of the modes of the external cavity.  
     [0050] Thus, given a plurality of resonant modes in an external cavity, the response of the gain cavity may be varied so as to select one or more of the external cavity&#39;s modes. For example, if it is desired to lase at a particular frequency, the response of M 2  may be configured to overlap only a single selected external cavity resonance, and any overlap should be of a low enough amplitude such that none of the immediately adjacent modes, or “neighbour” modes, will operate.  
     [0051] Or, if it is desired to have multiple frequencies lase, the reflectivity of M 2  may be lowered to select multiple resonances of the external cavity. The desired resonant modes of the external cavity may be characterized as a contiguous plurality of desired modes of operation interspersed in frequency between undesired modes of operation. Thus, by utilizing the advantageous disclosed methods, one can control the reflectivity of the mirror M 2  to select desired modes of operation.  
     [0052] It is contemplated that the reflectivity of M 3  may be adjusted to compensate for the reduced reflectivity of M 2 . For example, the reflectivity or other characteristics of M 3  might be varied based upon how much output power is desired.  
     [0053] The previous description of various embodiments, which include preferred embodiments, is provided to enable any person skilled in the art to make or use the present invention. The various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without the use of the inventive faculty. Thus, the present disclosure is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.