Abstract:
A multi-wavelength semiconductor laser is formed by monolithically integrating a plurality of laser diodes ( 1, 2 ) with at least one isolator section ( 3 ) and a coupler ( 4 ), which couples the different emission wavelengths λ 1 , λ 2  into one output port ( 5 ). The isolator section can be either a light absorptive type or wavelength selective type, including a Bragg grating type isolator or a photonic bandgap crystal type isolator. The coupler is preferably a Y-junction coupler, but can also be a multi-branch waveguide coupler or a waveguide directional coupler.

Description:
FIELD OF THE INVENTION 
   The present invention pertains to the field of semiconductor laser fabrication and in particular to a method for manufacturing integrated single output multiple wavelength lasers and related photonic integrated devices to be utilized in wavelength division multiplexing (WDM) optical communication system, optical recording and measurement 
   BACKGROUND OF THE INVENTION 
   Multi-wavelength laser sources are key elements for a variety of applications such as wavelength division multiplexing (WDM), optical recording, color displays and optical color printing. Several methods have been proposed and patented for the realization of these multi-wavelength laser sources and the integration with modulators, as can be seen in U.S. Pat. No. 4,955,030 (Menigaux), U.S. Pat. No. 4,831,629 (Paoli), U.S. Pat. No. 4,993,036 (Ikeda), U.S. Pat. No. 5,384,797 (Welch), U.S. Pat. No. 5,519,721 (Takano). They can be realized by the non-uniform current pumping resulting in areas with different band filling, selective area epitaxy or quantum well intermixing. The last two methods can also be used to fabricate the lasers integrated with modulators. 
   However, many of the multiple wavelength laser sources or arrays fabricated rely on a large spatial separation between different wavelength elements, and therefore complex focusing optics are required to align the individual lasers to the optical fiber. It is desirable that all the laser emissions are combined monolithically together on a wafer so that the alignment restrictions can be eliminated. One method uses serially aligned semiconductor lasers each with a directional coupler [see U.S. Pat. No. 5,233,187, Japanese Patent 05090715, European Patent 496348, Sakata]. An inevitable drawback of this method is that there cannot be many wavelengths integrated together otherwise the device will be too long and the internal loss will be large. Also the grating must be optimized to act both as a directional coupler and the wavelength selective reflector. Another method is to combine parallel aligned laser arrays into one output port by use of appropriate waveguide alignment, such as the Y-junction coupler, rib waveguide coupler structures etc [See Japanese Patent 24209, 58175884, IEEE Photon. Technol. Lett. Vol. 7, pp. 944-946, 1995]. One problem arising from the combination of different wavelengths together is that the cross talks among these wavelengths are inevitable without additional treatment. The longer wavelength laser element will be optically pumped to lasing even only when the shorter wavelength laser element is electrically biased. The consequence is that each wavelength is not independently addressable. One method is to add an electroabsorption type modulator to each laser element. This method is a little complicated since it needs a lot of additional electrodes to control each modulator. 
   In this patent we propose a new method, the introduction of an isolator, to provide the needed isolation among different wavelength laser elements. It does not need additional electrodes so it is simple to fabricate and easy to operate. We also provide the method of fabricating the said isolator and other integrated optoelectronic devices. 
   SUMMARY OF THE INVENTION 
   The invention has as its objective, to introduce an isolator or isolators monolithically integrated with the semiconductor laser arrays and to provide an easy method of producing integrated multiple wavelength semiconductor lasers that can independently emit each wavelength in one spot. 
   According to one aspect of the invention, there is provided A multi-wavelength semiconductor laser comprising:
         a plurality of laser diode sections in parallel arrangement and having different emission wavelengths relative to each other;   at least one isolator section, the or each of said isolator sections being integrated with a respective one of said laser diode sections;   a coupler integrated with the laser diode sections and the at least one isolator section to couple the different emission wavelengths into one output port.       

   The multi-wavelength semiconductor laser comprises three essential components: laser diode sections; at least one isolator section; and a coupler. The cavities of the laser diode sections form a parallel array to provide different emission wavelengths with each wavelength independently addressable. The coupler is used to couple the different wavelengths into one output port. The coupler may be, for example, a Y-junction coupler, a multi-branch waveguide coupler or a waveguide directional coupler such as a rib waveguide directional coupler. The isolator section is used to isolate the cross talks among the different wavelengths so as to ensure that the device can emit either each wavelength singly or their desired combinations. 
   The isolator section may be, for example, either a wavelength selective type or a light absorptive type. For the first type, it can be a distributed Bragg grating type isolator or a photonic bandgap crystal type isolator which is transmissive to one wavelength of the laser diode section but highly reflective to other wavelengths. For a dual wavelength laser source comprising two laser diode sections, an isolator may be integrated with one or each of laser diode sections and tuned to reflect the other wavelength which is not desired. For the second type of isolation, the isolator has a band gap energy transparent to the laser wavelength of the laser diode section but absorbing to all other laser wavelengths. There is no need for an electrical contact for the isolation region thus giving rise to ease of fabrication and operation. 
   In another embodiment, the multi-wavelength semiconductor laser comprises four laser diode sections, at least three of which are integrated with a respective isolator section. In this case, the coupler may include a series of Y-junction couplers coupled by intermediate elements or may include a four-branch coupler. 
   The integrated multi-wavelength laser source is advantageously fabricated using a quantum well based heterojunction semiconductor laser structure. The semiconductor substrate is preferably made of either GaAs or InP. The quantum well can be made of GaAs, InGaAs, InAlGaAs, or InGaAsP. The device can be made either by selective area metal organic vapor phase epitaxy (MOVPE) or by post growth quantum well intermixing technology. By controlling the area of the semiconductor laser surface covered by the intermixing inhibiting layer, such as the Ge layer, a one step selective area impurity free quantum well intermixing can be used to fabricate such devices. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1   a  is a schematic waveguide structure of a dual wavelength laser source with monolithically integrated Y-junction coupler and isolator.  FIG. 1   b  is a diagram of the device with ridge waveguide and vertical layer structure shown. 
       FIGS. 2   a  and  2   b  illustrate the band diagram and the experimental results of the relative photoluminescence spectra for the four different sections in the dual wavelength laser source after a one step selective area intermixing. 
       FIGS. 3   a  and  3   b  are schematic waveguide drawings for two types of four-wavelength laser sources with monolithically integrated Y-junction couplers and isolators. 
       FIG. 4  is a schematic waveguide drawing for a dual wavelength laser source with monolithically integrated Y-junction coupler and distributed Bragg gratings. 
       FIGS. 5   a  and  5   b  are schematic diagrams of a Y-junction coupler and a rib waveguide directional coupler. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1   a , there is shown a schematic waveguide structure of a dual wavelength laser source monolithically integrated with an isolator and a Y-junction coupler. It contains laser diode sections  1  and  2 , an isolator  3  and a Y-junction coupler  4 . The structure includes cleaved laser facets  5 . 
   The Y-junction coupler  4  is used to direct the two signals from the laser diode sections, (i.e. gain regions)  1  and  2  into a single output port. The length of the gain region and isolation region are 700 μm and 300 μm respectively. The Y-junction coupler  4  is composed of two S-bends with a radius of curvature of 1328 μm. The central space between the two branches is 34 μm. The bending loss in the Y-junction waveguide should be kept small in the design.  FIG. 1   b  shows the schematic diagram of a ridge waveguide and vertical layer structure. The ridge waveguide can be formed by either wet chemical etching or Plasma dry etching (ICP or RIB). The wafer is a graded index separate confinement heterojunction (GRINSCH) laser structure. The n-type substrate  7  can be either GaAs or IhP or other relevant semiconductors. A lower cladding layer  8  is provided with n-type doping to 2×10 18  cm −3  while an upper cladding layer  12  is provided with p-type doping to 5×10 18  cm −3 . There is also provided lower and upper confinement layers  9  and  11 , wherein  10  is the active region with a quantum well structure, and metal contacts  14  and  15  for the electrode. 
   The gain regions  1  and  2  have different wavelengths λ 1  and λ 2 . The Y-junction coupler region has a bandgap with an equivalent wavelength λ 4  that must be short enough to ensure that it is completely transparent to the laser wavelengths λ 1 , λ 2 . The isolator  3  has an equivalent wavelength λ 3  that should be low enough to be transparent to λ 1  but be absorbing to λ 2  so that the laser light from channel  2  will not optically pump channel  1  when only channel  2  is electrically biased. As a result, the two lasers can be operated either simultaneously or separately. Different band gap regions in the wafer can be realized by either selective area MOVPE or by a one step selective area-quantum well intermixing. 
     FIG. 2   a  shows a schematic band diagram illustrating the four different bandgap regions realized by quantum well intermixing. The energy levels in the quantum well are represented as numerals  22 ,  23 ,  24  and  25  for laser diode section  1 , isolator section  3 , laser diode section  2 , and Y-junction coupler section  4  respectively. Due to the interdiffusion of the well and barrier elements, the quantum well shape and composition changed thus leading to blueshift of the quantum well energy levels. The p and n type cladding layers are represented as  16  and  21  respectively. 
   Graded index layers  17  and  20  and separate confinement layers  18  and  19  are provided. Taking the InGaAs/GaAs laser sample with a single 8 nm wide In 0.2 Ga 0.8 As quantum well confined by two 10 nm GaAs barriers as an example,  FIG. 2   b  shows the low temperature photoluminescence spectra of the sample after using the technology of quantum well intermixing with a Ge interlayer controlling band gap tuning. By defining the area of the wafer covered with a Ge layer by varying the spacing between the 1 μm wide Ge strip array, different intermixing degree and hence, different emission wavelengths can be achieved. The Ge coverage for the four regions,  1 ,  2 ,  3 ,  4 , in this device are 100%, 45%, 25% and 0% respectively. The Ge strip arrays were formed by evaporating Ge onto the photo-resist patterned GaAs surface followed by lift-off. The whole sample surface was then covered by SiO 2  and undergo rapid thermal annealing. The as-grown sample has a peak wavelength at 916 nm or 1.353 meV. The peak position of the PL signals for Y-junction coupler  4  (0% Ge cover) are 77.8 meV and 52.4 meV larger than that of 1 (100% Ge cover) and 2 (45% Ge cover) respectively. The bandgap of isolator (25% Ge cover) is 43 meV larger than channel  1  and 18 meV larger than channel  2 . It is transparent to channel  1  but partly absorbing to channel  2 . 
     FIG. 3   a  and  FIG. 3   b  show schematic waveguide diagrams of a four-wavelength laser source emitting from one output port. Four different laser diode sections  26   1 ,  26   2 ,  26   3 , and  26   4  with wavelengths of λ 1 , λ 2 , λ 3 , λ 4 , respectively are provided as are isolator sections  27   1 ,  27   2 ,  27   3  and  27   4 . It is noted that the isolator that is in the path of the shortest wavelength laser diode section is not necessary. If we assume λ 1 &gt;λ 2 &gt;λ 3 &gt;λ 4 , then isolator  27   4  can be saved and replaced with the material of the same band gap as the Y-coupler. 
   In  FIG. 3   a , the four channels  26  are coupled by two Y couplers  28  to a set of two identical intermediate elements  29 , which in turn are coupled by Y coupler  30  to the output waveguide  31 . In  FIG. 3   b , the four channels are coupled by one coupler  33  to the output waveguide  35 . Some special treatment should advantageously be taken to this four-branch “Y” coupler, such as the employment of flank wings at the outmost two branches to ensure equal power division in the four branches. 
   As an example, we assume the energy level of the InGaAs/GaAs Quantum well structure sample, that is for  26   1  or λ 1 , is 1.360 eV. Then the energy levels for the other three branches of the laser diode sections  26   2  (λ 2 ),  26   3  (λ 3 ), and  26   4  (λ 4 ), with equal spaced wavelength difference of 12 meV, could be 1.372 eV, 1.384 eV, and 1.396 eV respectively. The energy level for the isolators  27   1 ,  27   2 , and  27   3  could be 1.39 eV, 1.402 eV and 1.414 eV. The band gap of the Y coupler region could be 1.440 eV. So the isolator  27   1 ( 27   2 ,  27   3 ) is transparent to laser diode  26   1  ( 26   2 ,  26   3 ) but is absorptive to the other three laser diodes  26   2 ,  26   3 , and  26   4  ( 26   1 ,  26   3 ,  26   4 ;  26   1 ,  26   2 ,  26   4 ). The wavelength spacings between the two adjacent gain regions can be changed within a range of 10 to 17 meV. These bandgap tuning can be realized by selective area quantum well intermixing carried out on the same wafer, as the method described above, or by selective area MOVPE. 
   Another type of isolators  38 ,  39  are shown in FIG.  4 . They are in the form of distributed Bragg gratings, photonic bandgap crystals, or a stack of layers that are designed to be transmissive only to the laser wavelength of their respective gain regions  36  and  37 . Here the distributed Bragg grating does not act as a reflector but as a wavelength filter. The two cleaved facets are still the two mirrors to construct the laser cavity. If the laser diode section  36 ,  37  are distributed feedback (DFB) laser or distributed Bragg reflecting (DBR) laser, then there will be two different pitch gratings along the light propagation direction. For a dual wavelength integrated laser source, isolators  38  and  39  can be a conventional distributed Bragg grating but be highly reflective to the laser wavelength of their opposite gain regions  37  and  36 , rather than  36  and  37 , respectively. Using the grating as the isolator, the wavelength difference between the adjacent gain regions can be made very small because of the very sharp wavelength selection characteristics of the gratings. Such an arrangement has a better performance in isolating the crosstalks among different laser wavelengths than the absorption type isolator. However it is more complicated in design and fabrication. 
   The Y-junction coupler  40  is a 3 port device arranged so that optical power fed into one port is divided equally between the other two ports. One physical form that such a coupler can take is that of a single mode waveguide  41  ( FIG. 5   a ) which divides at a small angle into two single mode waveguides  42 ,  43 . An alternative form is provided by a balanced directional coupler configuration in which the first waveguide  44  ( FIG. 5   b ) terminates in a region where it is symmetrically flanked on both sides by the start of two other waveguides  46 , 47 . The overlapping of the waveguides is such that power launched into the Y-junction coupler via waveguide  44  is fully coupled equally into the other two waveguides so that none is reflected at the end  45  of the first waveguide. The lateral separation of the two ports should be sufficient to provide no mutual evanescence coupling. 
   In the above explained embodiments, the practice of the invention has been demonstrated in the examples using GaAs type semiconductors. However, it can be effectively applied to all III-V type semiconductors. 
   While the invention has been described in its preferred embodiments, it is to be understood that the words which have been used are words of description rather than limitation and that changes may be made without departing from the true scope and spirit of the invention in its broader aspects.