Abstract:
There is described a high speed vertical-cavity surface-emitting laser (VCSEL) comprising a substrate and first and second distributed Bragg reflectors (DBRs) disposed on the substrate, each comprising a stack of layers of alternating refractive index. A resonant cavity is disposed between the DBRs and an active region disposed in the resonant cavity. The resonant cavity is formed of material having low refractive index and has an optical thickness in a direction perpendicular to the substrate of  ½ λ, where λ is the wavelength of light emitted by the VCSEL.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims benefit of British patent application number 1112927.7, filed Jul. 27, 2011, which is herein incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to high speed vertical-cavity surface-emitting lasers (VCSELs). 
         [0004]    2. Description of the Related Art 
         [0005]    In this specification the term “light” will be used in the sense that it is used in optical systems to mean not just visible light, but also electromagnetic radiation having a wavelength outside that of the visible range. 
         [0006]    The ever-increasing bandwidth-distance requirements of communication systems have resulted in data being transmitted over optical fibres. Both conventional telecommunications and data networks such as the Internet use optical fibres for both short and long distance transmission. Optical communication channels provide extremely high data rates (in excess of 10 Gbit/s or even 25 Gbit/s). Data that is to be sent down such channels is typically generated in the form of electrical signals that are converted to optical signals by directly modulating a laser at one end of an optical fibre. 
         [0007]    Vertical Cavity Surface Emitting Lasers (VCSELs) have become commercially important as transmitters in such high bit rate (&gt;1 Gbit/s) optical communication links. A VCSEL is a semiconductor laser device including one or more semiconductor layers (typically quantum wells) exhibiting an appropriate band gap structure to emit light in a desired wavelength range perpendicularly to the one or more semiconductor layers. Typically, the thickness of a corresponding semiconductor layer is in the range of a few nanometres. In the case of a multi-quantum well laser, the thickness and the strain created during the formation of the stack of semiconductor layers having, in an alternating fashion, a different gap, determine the position of the energy level in the quantum wells of the conduction bands and valence bands defined by the layer stack. The position of the energy levels defines the wavelength of the radiation that is emitted by recombination of an electron-hole-pair confined in the respective quantum wells. Unlike in edge emitting semiconductor laser devices, the current flow and the light propagation occurs in a vertical direction with respect to the semiconductor layers. Above and below the semiconductor layers respective mirrors, also denoted as top and bottom mirrors, wherein the terms “top” and “bottom” are exchangeable, are provided and form a resonator to define an optical cavity. The laser radiation established by the resonator is coupled out through that mirror having the lower reflectivity. 
         [0008]    Although VCSEL devices suffer from relatively low output power due to their small laser cavity, VCSELs are steadily gaining in importance in a variety of technical fields, since a VCSEL device exhibits a number of advantages when compared to a conventional double heterostructure laser diode, also referred to as edge-emitting lasers. First, a large number of VCSEL devices can be fabricated and entirely tested on the initial substrate, so that a significant reduction in manufacturing costs is obtained compared to edge-emitting lasers. Second, the overall volume of a single VCSEL device is reduced by a factor of about 10-100 compared to the double heterostructure laser diode. Third, due to the extremely small volume of the gain region that is defined in the vertical direction by the thickness of the semiconductor layers having in alternating fashion a different band gap, the current for operating the VCSEL device is in the range of a few milliamps, whereby a high efficiency of conversion of current into light is achieved. Fourth, a further VCSEL device exhibits a relatively low beam divergence, which allows a high coupling efficiency to other optical components, such as optical fibres, without the necessity of additional converging optical elements. 
         [0009]    Increasingly high speed VCSELS are required for high modulation at low currents. 10 Gbit/s VCSELs have become successful, but future needs for link capacities of 100 Gbit/s aggregate bandwidth create a demand for VCSELs capable of even higher modulation speeds. 
         [0010]      FIG. 1  illustrates the structure of a typical high speed VCSEL  100 . On a substrate  101  is an n-doped mirror  102  formed by alternating layers of high refractive index material  103  and low refractive index material  104  so as to produce a high reflectivity Distributed Bragg Reflector (DBR). A p-doped mirror  105 , also formed as a DBR by alternating high and low refractive index layers  103 ,  104 , is located above the n-doped mirror  102 , with a resonant cavity  106  formed therebetween. The cavity includes an active (gain) region  107  comprising one or more quantum well layers separated by barrier layers. An oxide layer  108  defining an aperture  109  is located between the cavity  106  and the p-mirror  105 . In this example it can be seen that the oxide layer  108  and p-mirror  105  form a mesa. The VCSEL  100  also includes a cap layer  110  on top of the p-mirror  105 . 
         [0011]    The cavity  106  usually has an optical thickness equal to the wavelength λ (or an integral number of wavelengths) of the light emitted by the laser. The material in the cavity generally has a low bandgap (and high refractive index) so there are many carriers.  FIG. 2A  illustrates the cavity  200  having a width of one wavelength.  FIGS. 2B and 2C  illustrate the bandgap  201  and refractive index  202 , respectively, of the cavity  200  (including the active layer) of  FIG. 2A . In general, the material  203  of the cavity on either side of the quantum well  204  (typically of width of the order 100-200 nm) acts as a “buffer” which reduces strain in the quantum wells. 
         [0012]      FIG. 3  illustrates the location of the electric field standing wave  301  and the bandgap  302  of a cavity of the type described above. 
         [0013]    VCSELs for wavelengths from 650 nm to 1300 nm are typically based on gallium arsenide (GaAs) wafers with DBRs formed from GaAs and aluminium gallium arsenide (Al x Ga (1−x) As). The GaAs—AlGaAs system is favoured for constructing VCSELs because the lattice constant of the material does not vary strongly as the composition is changed, permitting multiple “lattice-matched” epitaxial layers to be grown on a GaAs substrate. However, the refractive index of 
         [0014]    AlGaAs does vary relatively strongly as the Al fraction is increased, minimizing the number of layers required to form an efficient Bragg mirror compared to other candidate material systems. Furthermore, at high aluminium concentrations, an oxide can be formed from AlGaAs, and this oxide can be used to restrict the current in a VCSEL, enabling very low threshold currents. 
         [0015]    There is a need to increase the speed of high speed VCSELs such as those shown in  FIG. 1  even further. 
       SUMMARY OF THE INVENTION 
       [0016]    In accordance with one aspect of the present invention there is provided a high speed VCSEL comprising a substrate and first and second DBRs disposed on the substrate, each comprising a stack of layers of alternating refractive index. A resonant cavity is disposed between the DBRs and an active region disposed in the resonant cavity. The resonant cavity is formed of material having low refractive index and has an optical thickness in a direction perpendicular to the substrate of ½λ, where λ is the wavelength of light emitted by the VCSEL. The carrier delay in the cavity is 100 ps or less. 
         [0017]    In one embodiment the VCSEL may be fabricated using the AlGaAs/GaAs system, although it will be appreciated that other systems such as AlGaInIsP/GaAs, AlGaInNAsP/GaAs and InGaAsP/GaAs inter alia are also possible. At least one oxide layer may be provided in either or both of the DBRs. The cavity may be formed from Al doped material and optionally does not include any oxide. The Al material composition of the cavity is optionally at least 1% less than the Al composition of the oxide layer. The VCSEL may be configured to emit light modulated at rate of at least 10 Gbit/s, preferably at least 15 Gbit/s, more preferably at least 25 Gbit/s. The wavelength of light emitted may be in the range from 650 nm to 1.5 μm, and is optionally 850 nm. 
         [0018]    The distance between the active region and the low refractive index region of the cavity may be in the range 0 to 50 nm, preferably 0 to 25 nm, more preferably 5-15 nm. This may result in strain in quantum wells in the active region. 
         [0019]    The cavity may be disposed between two barrier layers, each barrier layer having a bandgap energy which is less than that of the cavity by a difference of 2 kT or greater, preferably 5 kT, 10 kT or 20 kT or greater. 
         [0020]    In accordance with another aspect of the present invention there is provided a method of generating modulated light, comprising injecting current into an active region of a VCSEL. The active region is located in a resonant cavity disposed between first and second DBRs. The cavity is formed of material having low refractive index and has an optical thickness of ½λ, where λ is the wavelength of light emitted by the VCSEL. The injected current is modulated at a rate of at least 5 Gbit/s, and optionally at 15 Gbit/s or higher. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
           [0022]      FIG. 1  is a schematic illustration of a prior art high speed VCSEL. 
           [0023]      FIGS. 2A ,  2 B and  2 C illustrate the thickness, bandgap and refractive index of a cavity of the VCSEL of  FIG. 1 . 
           [0024]      FIG. 3  is a schematic illustration of the bandgap and electric field distribution in a VCSEL similar to that shown in  FIG. 1 . 
           [0025]      FIG. 4  is a schematic illustration of an improved high speed VCSEL. 
           [0026]      FIGS. 5A ,  5 B and  5 C illustrate the thickness, bandgap and refractive index of a cavity of the VCSEL of  FIG. 4 . 
           [0027]      FIG. 6  is a schematic illustration of the bandgap and electric field distribution in a VCSEL similar to that shown in  FIG. 4 . 
           [0028]      FIG. 7  is a schematic illustration of an alternative structure of a λ/2 cavity VCSEL. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    The features which define a “high speed” VCSEL include one or more of the following:
       High-frequency modulation for opto-electronic components such as a laser &gt;1 GHz. Frequencies now 10 GHz, &gt;25 GHz   Low external parasitics: especially low Bondpad capacitance (&lt;1 pF) and low device resistance (&lt;100 Ohm)   Low internal parasitics of mesa, oxide layer, cavity   Sufficiently low mode volume and good carrier and optical mode confinement (typically by oxide aperture diameter &lt;15 μm)   Sufficiently high output power (order of magnitude mW)   Sufficiently high gain and differential gain (as for example in 8 nm GaAs/AlGaAs QWs, but also other materials could be used)   Sufficient separation of operating point current versus threshold current: Min 3× Ith, typical 5-10× Ith   Preferred operating point in the linear LI region   Small rise and fall times of optical response to electrical excitation (Example: 10 GBit/s: Rise time: 40 ps, Fall time: 45 ps)   Small photon and carrier lifetimes and low threshold current (typically &lt;2 mA)   Low power consumption (&lt;25 mW), high wall-plug efficiency (&gt;10%)   Preferred multi-mode operation (see U.S. Pat. No. 5,359,477)   Low spectral linewidth (nm range)   High reliability for commercial use (wear-out after several years under normal operating conditions)       
 
         [0044]    In order to overcome limitations which “slow down” the high speed operation of the VCSEL, it is proposed to reduce the cavity size to λ/2. A structure of a VCSEL having such a cavity is shown in  FIG. 4 . The structure of the VCSEL  400  includes a substrate  401  on which an n-doped mirror  402  is formed by alternating layers of high refractive index material  403  and low refractive index material  404  so as to produce a high reflectivity Distributed Bragg Reflector (DBR). A p-doped mirror  405 , also formed as a DBR by alternating high and low refractive index layers  403 ,  404 , is located above the n-doped mirror  402 , with a cavity  406  formed therebetween. The cavity includes an active (gain) region  407  comprising one or more quantum well layers separated by barrier layers. An oxide layer  408  defining an aperture  409  is located between the cavity  406  and the p-mirror  405 . In this example it can be seen that the oxide layer  408  and p-mirror  405  form a mesa. The VCSEL  400  also includes a cap layer  410  on top of the p-mirror  405 . 
         [0045]      FIG. 5A  illustrates the thickness of the cavity  406  of the VCSEL of  FIG. 4 . The cavity  406  includes the active region  407  sandwiched by two barrier layers  504 . A semiconductor material  504  is also formed immediately adjacent the barrier layers  504 . The barrier layers include high Al content. 
         [0046]      FIGS. 5B and 5C  illustrate the bandgap  502  and refractive index  503  of the cavity of the VCSEL of  FIG. 4 . The material used to form the cavity  406  is a relatively high bandgap, low refractive index material as shown in  FIGS. 5A-5C . This is thus an “inverted cavity” and an electric field distribution  601  with the bandgap  602  of such a cavity is shown in  FIG. 6 . 
         [0047]    It will be noted that, in  FIGS. 5A and 5B , the bandgap energy of each barrier layer  504  either side of the active region  407  of the cavity  406  is lower than that of the semiconductor  506  immediately adjacent to it. The difference in bandgap energy in the barrier  504  compared to the adjacent semiconductor  506  should be at least 2 kT, preferably 5 kT or more, and more preferably still 10 kT or more, where k is the Boltzmann constant and T is the temperature, which in most situations will be of the order of 300 K. This leads to a higher carrier confinement in the active region  407 . 
         [0048]      FIG. 7  is a schematic illustration of an alternative structure of a λ/2 cavity VCSEL. In this example the cavity  700  includes three active regions (quantum wells)  701   a,    701   b,    701   c  separated by two barrier layers  702   a,    702   b.  A narrow buffer layer  703  is also formed between the active regions  701   a,    701   b,    701   c  and a low index material  704  in the cavity  700 . It will be appreciated that the buffer layer  703  will typically be 10 nm or less in thickness. 
         [0049]    The use of a cavity of the type shown in  FIGS. 4 to 7 , in a VCSEL is unusual. Conventional edge-emitting lasers are formed as PIN junctions in such a way that the laser light is propagated through the Intrinsic (I) layer having a low band gap and high refractive index. VCSELs have traditionally been constructed on the same principle. However, because the cavity is grown rather than being part of the same layer that generates the light, it is possible to make a cavity from low refractive index material. 
         [0050]    In traditional VCSELs having λ cavities, the cavity length is typically 200-300 nm and made of intrinsic material. Because the refractive index is high, the mirrors are at wave anti-nodes (as shown in  FIG. 3 ). The use of a low refractive index material enables the mirrors to be at wave nodes (as shown in  FIG. 6 ) and it is this which enables the cavity to have length λ/2. 
         [0051]    A VCSEL is effectively a p-n junction, and a population inversion in the junction is needed to operate the device. In order to modulate the photonic field transmitted by the device, the carrier population in the cavity must be modulated. The field modulation responds to the carrier modulation in a manner similar to a coupled pendulum. 
         [0052]    For a high speed device a very fast response of the photonic field is required. A λ/2 high bandgap/low refractive index cavity has fewer carriers than a conventional A cavity, and this makes it possible to modulate the carriers much more quickly (because there are fewer electrons/holes created under lasing conditions and fewer holes to fill when lasing is switched off). 
         [0053]    It will be noted that, in most other respects the high speed VCSEL shown in  FIG. 4  is similar to that shown in  FIG. 1 . In particular, the materials of the cavity are not oxidised. This allows the increase in speed of the VCSEL to come about without a corresponding decrease in reliability. If the VCSEL is based on the AlGaAs/GaAs system, the material composition of the cavity needs to be at least 1% less than the oxide layer composition. 
         [0054]    The use of an inverted λ/2 cavity results in faster carrier transport, an enhanced interaction between photon and carrier populations in the active region, and a lower photon lifetime compared to a conventional high speed VCSEL. These effects make the VCSEL faster (higher modulation bandwidth). The low refractive index material in the cavity has a higher energy gap with also helps to confine the carriers (especially electrons) in the active region. This also improves the temperature behaviour of a VCSEL. This type of VCSEL design therefore has significant advantages in a high frequency datacom VCSEL. 
         [0055]    There are other advantages displayed in a λ/2 cavity. The long “buffer”  203  between the area with high bandgap (and Al content) and the quantum wells shown in  FIGS. 2A and 2B  is no longer present. Instead, the distance between the edge of the active region (quantum wells)  407  and the region having high Al content  504  (and thus high bandgap and low refractive index) may be 10 nm or even less on both sides of the active region (shown in  FIGS. 5A and 5B ). This introduces strain into the quantum wells and this results in further improvements in the speed of the device. 
         [0056]    In a traditional λ cavity (shown in  FIGS. 2A to 2C ), all of the high refractive index material  203  which forms the cavity has a low Al content, resulting in a low lattice mismatch to the quantum well  204 , and it is this which enables the cavity material to act as a stress relief buffer layer. In a GaAs system this means that there is a layer of 120 nm or more between the active region  204  and the “next” high AlGaAs. On the contrary, in a λ/2 cavity (shown for example in  FIGS. 5A to 5C ), this buffer layer with its low lattice mismatch is not present and the active region  407  is sandwiched between two high AlGaAs layers  504  with a gap of 10 nm or less. 
         [0057]    By using a λ/2 cavity it is possible to increase the VCSEL bandwidth by more than 5 GHz using the same active region material (GaAs) as in a previous lambda cavity design. This design shows good reliability and high speed. It may also increase the bandwidth of VCSELs with a strained active region, such as for example InGaAs. It will be appreciated that the quantum wells in the active region may be formed from GaAs, AlGaAs, GalnAs, AlGaInAsP, or similar materials. 
         [0058]    If the device is built on a GaAs substrate it is possible to produce wavelengths in the range from about 650 nm to about 1.5 μm, depending on the material used in the active region. In general, 850 nm is the preferred wavelength for fibre communication. 
         [0059]    Thus the arrangements described above include the following features compared to a conventional VCSEL:
       Lower active volume in the inverted λ/2 cavity   Reduction of carrier transport delay to quantum wells   Reduction of active region carrier number outside the quantum well (=buffered carriers). This can lead to elimination of the buffer layer   Higher electric field overlap with quantum wells: higher stimulated emission rate, lower threshold   Lower photon lifetime leading to less damping and higher speed       
 
         [0065]    While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.