Abstract:
A multi-quantum well optical waveguide structure comprises a plurality of active regions including quantum wells with different gain peak wavelengths to provide an ultra broadband optical gain spectrum. Two adjacent sets of active regions having a large band gap difference are connected by a tunneling injection layer to provide smooth electron transport. Single transverse-mode operation is obtained by narrowly tapering the width of the multi-quantum well optical waveguide from the center to the two ends. Higher-order modes are suppressed at the output of the tapered waveguide, even though the center waveguide portion supports higher-order modes. In this way, the multi-quantum well optical waveguide can be utilized for ultra broadband optical amplification using a single mode fiber.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an optical waveguide to produce an ultra broadband optical gain which can be used for broadband optical amplifiers and ultra-broadly tunable lasers in optical wavelength division multiplexing (WDM) network systems. 
     2. Description of Related Art 
     Semiconductor optical amplifiers and tunable lasers need broad band gain in order to fully utilize the advantage of optical WDM network systems. Quantum wells have been used in these devices to provide higher and broader optical gain. Multi-quantum (MQW) wells consisting of an identical composition and thickness can provide a typical gain spectrum of 60-70. Conventional methods have been used to broaden the gain spectrum beyond this range, see S. Ikeda et al., “Semiconductor laser devices with a plurality of light emitting layers having different bands gaps and methods for driving the same” IEEE Photonics Technology Letters, Vol. 16, No. 2, February 2004, and U.S. Pat. No. 5,224,114. Ikeda et al. describe light emitting layers of two different quantum wells having different band gaps which provide two different gain peak wavelengths. Since the total gain spectrum is a sum of the two gain spectra, the gain spectrum is broader than that obtained by using one type quantum well. Ikeda et al. describe AlGaAs/GaAs materials which have a wavelength range around 850 nm. However, Ikeda et al. has the following disadvantage. There is a barrier layer between the two different quantum wells, which has a larger band gap than the band gap of the two quantum wells. In order for the barrier layer to confine the carriers in the two adjacent quantum wells, the band gaps of the two quantum well materials cannot be too different because the carrier injection efficiency degrades. 
     In current WDM metro and long-haul optical communications systems, 1.3 μm and 1.5 μm wavelength ranges are used. In these wavelength ranges, a broad optical gain spectrum ranging from 1.3 μm to 1.5 μm has been required. C. C. Huang et al., “174-nm mode spacing in dual-wavelength semiconductor laser using non-identical InGaAsP quantum wells”, IEEE Photonics Technology Letters, vol. 16, No. 2, pp. 371-373, February 2004. Using InGaAsP and InGaAs quantum wells to provide gain peak wavelengths of around 1.3 μm and around 1.5 μm, respectively. Accordingly, a broad gain spectrum range from 1356 nm to 1530 nm is achieved. However, Huang et al. has the following disadvantage. In order to broaden the gain spectrum band, the difference of the band-gaps of the two quantum wells materials needs to be larger. Since the barrier layer of InGaAsP is used for the two quantum wells, the larger band gap difference in the two quantum wells causes a degradation of carrier injection efficiency, resulting in the reduction of the magnitude of the gain. 
     It is desirable to provide an optical waveguide with improved higher and broader optical gain. 
     SUMMARY OF THE INVENTION 
     A multi-quantum well optical waveguide structure having quantum wells of different gain peak wavelengths provides an ultra broadband optical gain spectrum. A tunneling junction layer is disposed between two adjacent quantum wells having a large bandgap difference for making smooth electron transport. The multi-quantum well optical waveguide provides an ultra broadband optical gain to cover the full fiber transmission window from about 1200 nm to about 1700 nm (bandwidth of about 500 nm) and beyond. 
     In one embodiment, single transverse-mode operation is obtained by narrowly tapering the width of the multi-quantum well optical waveguide from the center to an end or the two ends to suppress higher-order transverse mode excitation 
     The invention will be more fully described by reference to the following drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a multi-quantum well optical waveguide structure in accordance with an embodiment of the invention. 
         FIG. 2  is a schematic band energy diagram of the multi-quantum well optical waveguide structure shown in  FIG. 1 . 
         FIG. 3  is a measured gain spectrum of the multi-quantum well optical waveguide structure shown in  FIG. 1 . 
         FIG. 4A  is a measured output power versus current relation of the laser when the bias is set above a threshold. 
         FIG. 4B  is the laser output spectrum. The lasing wavelength is at the middle of the two-gain region. 
         FIG. 5  is a schematic diagram of a multi-quantum well optical waveguide structure in accordance with an alternate embodiment of the present invention. 
         FIG. 6  is a schematic band energy diagram of the multi-quantum well optical waveguide structure shown in  FIG. 5 . 
         FIG. 7  is a schematic diagram of the multi-quantum well optical waveguide structure equipped with a tapered waveguide in accordance with another embodiment of the invention. 
         FIG. 8  is a schematic diagram of the multi-quantum well optical waveguide structure used for mode profile simulation. 
         FIG. 9A  is a simulated mode profile of the waveguide structure in  FIG. 8  at z=100 μm. 
         FIG. 9B  is a simulated mode profile of the waveguide structure in  FIG. 8  at z=400 μm. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in greater detail to a preferred embodiment of the invention, an example of which is illustrated in the accompanying drawings. Wherever possible, the same reference numerals will be used throughout the drawings and the description to refer to the same or like parts. 
       FIG. 1  is a schematic diagram of a multi-quantum well waveguide transverse structure  10  in accordance with the teachings of the present invention. Multi-quantum well waveguide transverse structure  10  comprises first active region  12  and second active region  14 . Tunneling junction layer  16  is disposed between first active region  12  and second active region  14  making injection efficiency higher with transporting electrons through the whole quantum well regions. First active region  12  can comprise a plurality of quantum wells for providing a predetermined gain peak wavelength. For example, first active region  12  can comprise six quantum wells having a predetermined thickness for providing a gain peak wavelength of 1.45 μm. In this embodiment, first active region  12  comprises six quantum wells of about 5 nm thick InGaAsP with 1.45 μm bandgap wavelength and five barriers of about 8 nm thick InGaAsP with 1.5 μm bandgap wavelength. Second active region  14  can comprise a plurality of quantum wells for providing a predetermined gain peak wavelength. For example, second active region  14  can comprise six quantum wells having a predetermined thickness for providing a gain peak wavelength of 1.55 μm. In this embodiment, second active region  14  comprises six quantum wells of about 6 nm thick InGaAs with 1.55 μm bandgap wavelength and five barriers of about 8 nm thick InGaAsP with 1.1 μm bandgap wavelength. 
     In one embodiment, n-InP buffer layer  22  is provided on n-InP substrate  21 . n-InP buffer layer  22  can have a thickness of about 0.139 μm. InGaAsP layer  23  is disposed on n-InP buffer layer  22 . InGaAsP layer  23  can have a thickness of about 0.1 μm to provide a 1.1 μm bandgap wavelength. First active region  12  is disposed on InGaAsP layer  23 . InGaAsP layer  24  is disposed on first active region  12 . InGaAsP layer  24  can have a thickness of about 0.1 μm to provide a 1.1 μm bandgap wavelength. p-InP layer  25  is disposed on InGaAsP layer  24  as a doping layer. p-InP layer  25  can have a thickness of about 60 nm. 
     Tunneling junction layer  16  can be diposed on p-InP layer  25  which doping level is graded from low to high. For example, tunneling junction layer  16  comprises p+-InP layer  26  (with Zn as p-dopant), p+-InAlAs layer  27  (with carbon as p-dopant), and n+-InP layer  28  (with Si as n-dopant). 
     p-InP layer  29  can be disposed on tunneling junction layer  16 . p-InP layer  29  acts as a doping layer and can have a thickness of about 60 nm. InGaAsP layer  30  is disposed on p-InP layer  29 . InGaAsP layer  30  can have a thickness of about 0.1 μm to provide a 1.1 μm bandgap wavelength. Second active region  14  is disposed on InGaAsP layer  30 . InGaAsP layer  31  is disposed on second active region  14 . InGaAsP layer  31  can have a thickness of about 0.1 μm to provide a 1.1 μm bandgap wavelength. 
     InP layer  32  can be disposed in InGaAsP layer  31 . InP layer  32  can have a thickness of about 0.2 μm. p-InP layer  33  can be disposed on InP layer  32 . p-InP layer  33  can be a graded doping layer having a thickness of about 0.3 μm. p-InP layer  34  can be disposed on p-InP layer  33 . p-InP layer  34  can have a thickness of about 1.1 μm. p-InGaAs contact layer  35  can be disposed on p-InP layer  34 . p-InGaAs layer  35  can be a contact layer having a thickness of about 0.16 μm. p metal layer  36 , such as Ti—Pt—Au, is disposed on p-InGaAs layer  35 . n metal layer  37 , such as Au—Ge, can be disposed on n-InP substrate  21 . 
       FIG. 2  illustrates a band diagram of multi-waveguide structure  10  showing tunneling junction layer  16  and first active region  12  which have different band gap energies. Variable thickness of the quantum wells described above can be used to spread the gain peak wavelengths. The band gap wavelength of the first set of the multi-quantum well layers of first active region  12  (on the left-hand side of the diagram) varies from λ 1  to λ 4 , and the bandgap wavelength of the second set of the multi-quantum well layers at the second active region  14  (on the right-hand side of the diagram) from λ 5  to λ 8 . In this way, the broad gain spectrum ranging from λ 1  to λ 8  can be generated. The large band-gap wavelength difference between λ 4  to λ 5  is connected smoothly by tunneling junction layer  16 . 
       FIG. 3  is a measured gain spectrum of multi-quantum well waveguide transverse structure  10  shown in  FIG. 1 . The gain profile is not smooth in first active region  12  because all quantum wells in each p-i-n section are the same size. 
       FIG. 4A  shows the measured output power versus current relation of the laser when the bias is set above a threshold.  FIG. 4B  shows the laser output spectrum. The lasing is obtained at the middle of the two peak wavelengths of the two gain regions. 
     With the presence of multiple gain materials connected by tunnel junctions, it has been found that it is difficult to achieve single mode waveguides since the layer thickness in the vertical direction is large, for example, in the range of greater than about 4000 nm thick. It has been found that using a waveguide width of about 1 um wide waveguide in the horizontal direction obtains a multi-mode waveguide. 
       FIG. 5  illustrates an alternate embodiment of the present invention. Multi-quantum well waveguide transverse structure  40  comprises five different sets of active layers  42   a - 42   e  each with different gain peak wavelengths disposed on substrate  41 . For example, the five sets of active layers  42   a - 42   e  can have gain peak wavelengths of 1200 nm for active layer  42   a,  1300 nm for active layer  42   e,  1400 nm for active layer  42   d,  1500 nm for active layer  42   b , and 1600 nm for active layer  42   c . Each set of adjacent active layers  42   a - 42   e  is connected to tunneling layer  43 . 
     An example embodiment of multi-quantum well waveguide transverse structure  40  comprises active layer  42   a  formed of about 0.05 μm thick InGaAsP layer with a 1.05 μm bandgap wavelength, about 0.07 μm thick InGaAsP layer with 1.2 μm bandgap wavelength, about 0.05 μm thick InGaAsP layer with 1.05 μm bandgap wavelength, and about 0.1 μm thick p-InP.layer (p-type dopant is, for example, Zn). Active layer  42   b  is formed of about 0.1 μm thick n-InP layer, (n-type dopant is, for example, Si) about 0.05 μm thick InGaAsP layer having a bandgap wavelength of 1.05 μm, about 0.07 μm thick InGaAsP layer having a bandgap wavelength of 1.05 μm, about 0.07 μm thick InGaAsP layer having a bandgap wavelength of 1.50 μm, about 0.05 μm thick InGaAsP layer having a bandgap wavelength of 1.05 μm, and about 0.1 μm thick p-InP layer. Active layer  42   c  is formed of about 0.1 μm thick n-InP layer, about 0.05 μm thick InGaAsP layer having a bandgap wavelength of 1.05 μm, about 0.07 μm thick InGaAsP layer having a bandgap wavelength of 1.6 μm, about 0.05 μm thick InGaAsP layer having a bandgap wavelength of 1.05 μm, and about 0.1 μm thick layer of p-InP. Active layer  42   d  is formed of about 0.1 μm thick n-InP layer, about 0.05 μm thick InGaAsP layer having a bandgap wavelength of 1.05 μm, about 0.07 μm thick InGaAsP layer having a bandgap wavelength of 1.4 μm, about 0.05 μm thick InGaAsP layer having a bandgap wavelength of 1.05 μm and about 0.1 μm thick p-InP layer. Tunneling layer  43  can comprise, for example, C—InAlAs and n-InP. 
     Active layer  42   e  is formed of about 0.1 μm thick n-InP layer, about 0.05 μm thick Q layer with a 1.05 μm bandgap wavelength, about 0.07 μm thick InGaAsP layer with a 1.30 μbandgap wavelength, about 0.05 μm thick InGaAsP layer with a 1.05 μm bandgap wavelength and a cap region including a 1.5 μm thick p-InP layer and a 150 nm thick p-InGaAs layer. 
       FIG. 6  shows a band diagram of the whole layer structure which comprises five different sets of active layers shown in  FIG. 5 . The gain peak wavelengths of the first set of active layers to the fifth set of active layers vary from λ 1  to λ 5 . In this way, the broad gain spectrum ranging from λ 1  to λ 5  can be generated. The large band-gap wavelength difference between λ 1  to λ 5  is connected smoothly by the tunneling junction layers. Without tunneling junction layers, the carrier injection into the whole structure is very difficult due to large band barriers for electrons and holes. 
     An alternate embodiment of the invention is shown in  FIG. 7 , which provides a method to achieve single mode operations, which are useful for most broadband optical communication applications. The approach is to make narrow gradually the width of the waveguide in the horizontal direction to provide a tapered waveguide. In this way, higher-order modes are suppressed for excitation and only a fundamental transverse mode is maintained. Tapered waveguide  50  includes laser section  52  and tapered waveguide section  54 . For example, laser section  52  can have a length of about 100 μm and tapered waveguide section  54  can have a length of about 500 μm. Laser section  52  has a width of about 1.5 μm which gradually tapers to point section  55  of tapered waveguide section  54 . Both the higher transverse modes excited by a transverse thickness larger than the lowest transverse (vertical) mode thickness and the higher lateral (horizontal) modes excited by a lateral width larger than the lowest lateral mode width, will decay as they propagate along the horizontal direction due to their larger propagation loss compared to the propagation loss of the lowest mode. 
     In a typical amplifier operation, input signal from one side of the amplifier waveguide will adiabatically expand its transverse mode to cover the full cross section of the large multimode waveguide without generating higher order modes. It is possible that there may be a small amount of energy transferred to the higher mode due to certain non-uniformity along the waveguide. The majority of the modal energy will stay at the fundamental mode if the adiabatic mode expander if the waveguide quality is uniform. When the signal propagates to the output side of the amplifier waveguide, the output adiabatic mode expander serves as the modal filter to transfer the fundamental mode signal back to the smaller cross section single mode waveguide and strip off the residue higher order modes. 
       FIG. 8  is a schematic diagram of multi-quantum well waveguide transverse structure  60  of an alternate embodiment. 
     Multi-quantum well waveguide transverse structure  60  comprises first active region  62  and second active region  64 . Tunneling junction layer  66  is disposed between first active region  62  and second active region  64  making injection efficiency higher with transporting electrons through the whole quantum well regions. First active region  62  can comprise a plurality of quantum wells for providing a predetermined gain peak wavelength. For example, first active region  62  can comprise six quantum wells having a predetermined thickness for providing a gain peak wavelength of 1.3 μm. In this embodiment, first active region  62  comprises six quantum wells of about 5 nm thick InGaAsP with 1.3 μm bandgap wavelength and five barriers of about 8 nm thick InGaAsP with 1.1 μm bandgap wavelength. Second active region  64  can comprise a plurality of quantum wells for providing a predetermined gain peak wavelength. For example, second active region  64  can comprise six quantum wells having a predetermined thickness for providing a gain peak wavelength of 1.55 μm. In this embodiment, second active region  64  comprises six quantum wells of about 6 nm thick InGaAs with 1.55 μm bandgap wavelength and five barriers of about 8 nm thick InGaAsP with 1.1 μm bandgap wavelength. 
     In one embodiment, n-InP buffer layer  72  is provided on n-InP substrate  71 . n-InP buffer layer  72  can have a thickness of about 0.139 μm. InGaAsP layer  73  is disposed on n-InP buffer layer  72 . InGaAsP layer  73  can have a thickness of about 0.1 μm to provide a 1.1 μm bandgap wavelength. First active region  62  is disposed on InGaAsP layer  73 . InGaAsP layer  74  is disposed on first active region  62 . InGaAsP layer  74  can have a thickness of about 0.1 μm to provide a 1.1 μm bandgap wavelength. p-InP layer  75  is disposed on InGaAsP layer  74  as a doping layer. p-InP layer  75  can have a thickness of about 0.2 μm. 
     Tunneling junction layer  66  can be disposed on p-InP layer  75 . For example, tunneling junction layer  66  can have a thickness of about 13 nm. Tunneling junction layer  66  can comprise InAlAs layer  76 , n-InP layer  77  and n-InP layer  78 . For example, InAlAs layer  76  can have a thickness of about 13 nm, n-InP layer  77  can have a thickness of about 2 nm, and n-InP layer  78  can have a thickness of about 13 nm. 
     p-InP layer  79  can be disposed on tunneling junction layer  66 . p-InP layer  79  acts as a doping layer and can have a thickness of about 0.2 μm. InGaAsP layer  80  is disposed on p-InP layer  79 . InGaAsP layer  80  can have a thickness of about 0.1 μm to provide a 1.1 μm bandgap wavelength. Second active region  64  is disposed on InGaAsP layer  80 . InGaAsP layer  81  is disposed on second active region  64 . InGaAsP layer  81  can have a thickness of about 0.1 μm to provide a 1.1 μm bandgap wavelength. 
     InP layer  82  can be disposed in InGaAsP layer  81 . InP layer  82  can have a thickness of about 0.2 μm. p-InP layer  83  can be disposed on InP layer  82 . p-InP layer  83  can be a graded doping layer having a thickness of about 0.3 μm. p-InP layer  84  can be disposed on p-InP layer  83 . p-InP layer  84  can have a thickness of about 1.1 μm. p-InGaAs contact layer  85  can be disposed on p-InP layer  84 . p-InGaAs layer  85  can be a contact layer having a thickness of about 0.16 μm. p metal layer  86 , such as Ti—Pt—Au, is disposed on p-InGaAs layer  85 . n metal layer  87 , such as Au—Ge, can be disposed on n-InP substrate  71 . 
     A mode profile simulation has been conducted for the embodiment shown in  FIG. 8 . The simulated mode profiles at z=100 μm and z=400 μm are shown in  FIGS. 9A and 9B , respectively. 
     It is to be understood that the above-described embodiments are illustrative of only a few of the many possible specific embodiments, which can represent applications of the principles of the invention. Numerous and varied other arrangements can be readily devised in accordance with these principles by those skilled in the art without departing from the spirit and scope of the invention.