Abstract:
A multisegment laser diode structure is presented in the form of two spaced-apart linear waveguide segments and two spaced-apart ring-like waveguide segments, arranged such that each of the ring-like segments is optically coupled to each of the linear waveguide segments. At least one of the waveguide segments includes an active lasing material. The waveguide segments are thus arranged such that four separate electrical contacts can be provided to four waveguide segments, respectively, thereby enabling separate driving of each of the waveguide segments.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention is generally in the field of integrated lasers based on diode lasers, and relates to a multisegment integrated laser and a method for its manufacture. The invention is particularly useful for optical communications and related applications, such as tunability, modulation, amplification, wavelength conversion, and others.  
         BACKGROUND OF THE INVENTION  
         [0002]    Laser diodes have a vast field of applications and they are propelling the optical communications field. This is due to their small size, relatively simple construction and high reliability. In order to support higher functionality of such light sources, many configurations, in which an “on-chip” laser is composed of or attached to multiple functional segments, have been introduced in recent years. The following are some examples of the known light sources of the kind specified:  
           [0003]    (1) C 3  laser, where laser cavities are monolithically cascaded (U.S. Pat. No. 4,622,671);  
           [0004]    (2) DBR laser, where the active segment(s) is(are) sandwiched between two gratings segments (Y. Suematsu, S. Arai and K. Kishino, “ Dynamic Single Mode Semiconductor Laser with a Distributed Reflector”, IEEE J of Lightwave Tech. LT- 1, 161 (1983));  
           [0005]    (3) Master oscillator power amplifier (MOPA), where the laser segment is attached to a power amplifier segment (U.S. Pat. No. 5,126,876); and  
           [0006]    (4) a monolithically integrated externally modulated laser, where the laser segment is attached to an external modulator segment (U.S. Pat. No. 5,548,607)  
           [0007]    All the above prior art configurations suffer from crucial difficulties in their manufacture and in the possibility of extending their functionality. One of the key problems is to generate an on-chip laser mirror, in which the functionally different segments of the laser are separated from each other. Various technologies have been suggested to implement this idea, such as on-chip etching or cleaving, or using sub-micrometer period gratings. However, due to the technological difficulties, all existing devices of the kind specified, such as tunable integrated sources, MOPA, monolithically mode locked lasers and monolithically externally modulated lasers, are difficult to fabricate and thus expensive.  
         SUMMARY OF THE INVENTION  
         [0008]    There is accordingly a need in the art to facilitate the manufacture and operation of integrated lasers by providing a novel multisegment laser diode and method for its manufacture, to enhance the functionality of the laser by providing separation between the functionally different segments of the laser.  
           [0009]    The present invention utilizes the advantage of the concept of a complex cavity (resonator) based on two segments of waveguides inter-coupled by at )east two integrated ring microcavities. This technique has been developed by the inventors of the present application and is disclosed in WO 01/27692, assigned to the assignee of the present application. The main idea of the present invention is based on utilizing such a configuration of a complex cavity as the core technology for producing multisegment functional lasers. According to his concept, the laser mirrors are ring microcavities which can be of very high quality, thus enabling the implementation of mirrors with a desired reluctance, while as cleaved or as etched mirrors of the prior art techniques are limited to approximately 30% reflectance, This facilitates the generation of ultrashort cavity lasers. The ring mirror is wavelength sensitive, and can thus be used for both stabilizing the laser frequency and tuning it by externally changing the ring parameters.  
           [0010]    The unique feature of the above configuration consists of that light reflected from a ring mirror enters a channel (waveguide) different from that of die incident light propagation. Hence, an additional segment is generated in the laser cavity that can be exploited for an additional functionality (e.g., active mode locking, internal EA modulation, etc.). The ring mirror based technology does not require sub-micron resolution (i.e., about 0.2 micron for the DBR gratings). The ring mirror is relatively small (e.g., at the order of 5-50 μm in diameter). The ring mirror is at least a four-port coupler, which can be further used to enhance tie functionality and inter-chip connectivity of the multisegment lasers. All the segments (linear and ring waveguides) are buried and electrodes are deposited such that each of the segments can be manipulated separately (by the injection of a current or by applying external fields).  
           [0011]    Since optical waveguides can be implemented in a complex manner, the universal quantity characterizing the behavior of the confined light is the effective refractive index of the waveguide. In conventional passive devices, the difference between the effective refractive index of the waveguide and the index of the surrounding medium is typically smaller than 1%. In a semiconductor diode laser, the effective index difference is about 10%. When using ring micro resonator structures (with a small radius), the effective refractive index of the ring waveguide has to be relatively large, i.e., typically in the range of 10%-20% (depending on the ring diameter), to accommodate tight mode confinement and small losses.  
           [0012]    It should be understood that the terms “ring” or “ring-like” used herein signify any sufficiently smooth stricture of a closed-loop or ring-like shape, such as elliptical, stadium-like. etc., and not necessarily a circular shape. Thus, according to one aspect of the present invention, there is provided a multisegment laser diode structure comprising two spaced-apart linear waveguide segments and two spaced-apart ring-like waveguide segments carried on a substrate, each of the ring-like waveguide segments being coupled to each of the linear waveguide segments, wherein at least one of the waveguide segments includes an active lasing material, the segments&#39; arrangement enabling separate electrical contact to each of the segments, thereby enabling separate operation of each of the segments by current injection or application of an external field.  
           [0013]    According to one embodiment of the invention, each of the waveguide segments is made of an active lasing material. In this case., the segments are produced either by the dry etching of a semiconductor layer structure, as typically employed in the conventional ridge-based laser manufacture, or by a planar process based on proton implantation within tie areas surrounding the segments to generate a gain-guiding laser.  
           [0014]    The specific layer structure, as well as the physical separation between the ring-like and linear waveguide segments, is designed so as to provide the required optical coupling between the segments. Each of the segments call be used for modulation, tuning, q-switching and mode locking.  
           [0015]    According to another embodiment of the invention, only the linear waveguide segments contain the laser active material, and are defined by conventional dry etching. Subsequently, a deep etch trench (below the active layer) is etched in between the two linear ridge segments. Then the inter-laser spacing is covered by a thin low index layer, and subsequently by a high index layer, in which the ring microcavities are defined and subsequently buried. The ring segments can be used to tune the laser (e.g., by the thermooptic effect).  
           [0016]    The above cavities may be coupled to external segments either horizontally or vertically (for miniaturization) through one or the two rings (e.g., to a power amplifier or external modulator, or to a passive waveguide).  
           [0017]    According to another broad aspect of the invention, there is provided a method for manufacturing a multisegment laser diode structure the method comprising:  
           [0018]    (a) growing a semiconductor structure with a p-n junction between two semiconductor layers and an active laser layer within the junction;  
           [0019]    (b) patterning said semiconductor structure and carrying out layer deposition processes, to define two spaced-apart linear metalized ridge segments and two spaced-apart ring-like metalized ridge segments, the segments&#39; arrangement providing desired optical coupling of each of the ring-like segments to each of the linear segments, each of the ridge segments being formed with a metal layer on top thereof;  
           [0020]    (c) depositing a burying layer onto a structure obtained in step (b), allowing for contact windows above each of the segments; and  
           [0021]    (d) carrying out a further metal deposition to form four electrical contacts with the metal layer on top of the four segments, respectively.  
           [0022]    The patterning of the semiconductor structure may comprise patterning of the top layer thereof to define both the linear waveguide segments and the ring waveguide segment, all the segments thereby containing an active lasing material. Alternatively, Me patterning of the semiconductor structure may include patterning of the top layer thereof to define the spaced-part linear waveguide segments, and patterning (etching) of the top and active laser layer underneath the top layer, or all three layers of the semiconductor structure within the space between the linear waveguides, thereby defining an interlaser spacing. In this case, the two ring waveguide segments are defined in a high refraction index layer deposited within the interlaser spacing on top of a low refraction index layer (buffer layer).  
           [0023]    According to yet another aspect of the present invention, there is provided an integrated optical device comprising a multisegment laser diode structure constructed as described above, and an additional waveguide segment optically coupled to at least one of said linear or ring-like waveguides. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    In order to understand the invention and to see how it may be carried out in practice, a preferred embodiment will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:  
         [0025]    FIGS.  1  to  3  illustrate the main stages in the manufacture of a laser structure according to one embodiment of the invention, wherein FIG. 1 illustrates a structure after the dry etch step; FIG. 2 illustrates the structure of FIG. 1 after tie deposition of a burying layer and opening contact windows; and FIG. 3 illustrates the structure of FIG. 2 after deposition of the electrode;  
         [0026]    [0026]FIG. 4 illustrates a cross-section of the structure of FIG. 3 taken along line A-A;  
         [0027]    [0027]FIGS. 5A and 5B illustrate top and cross sectional views, respectively, of a laser structure according to another embodiment of the invention;  
         [0028]    [0028]FIG. 6 illustrates a laser structure according to yet another embodiment of the invention; and  
         [0029]    [0029]FIGS. 7A and 7B illustrate two more examples, respectively, of a structure according to the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]    Referring to FIGS.  1 - 3 , there are illustrated main operational steps in the manufacture of a multisegment laser diode structure according to one example of the inventions namely, a laser of the index-guide type typically comprising an active region interposed between n- and p-type semiconductor layers grown on a substrate.  
         [0031]    [0031]FIG. 1 shows a structure  10  obtained by such main processes of integrated technology as deposition, metalization and ridge etching. Thus, the structure  10  is composed of a substrate layer L 0  (i.e., made of GaAs or InP) carrying a laser layer-structure L 1 , which is formed by sequentially depositing an n-type semiconductor layer L 1 , an active laser layer L 3 , and a p-type semiconductor layer L 4  to define a p-n junction region between the semiconductor layers. When using the GaAs substrate L 0 , the n-type layer L 2  is AlGaAs doped with Si, layer L 3  is undoped AlGaAs containing several quantum wells of GaAs or InGaAs, and the p-type layer L 4  is another AlGaAs doped with Be or C. When using die InP substrate L 0 , the n-type layer L 2  is InP/InGaAsP doped with n-dopant, layer L 3  is the laser active layer made of several quantum wells of InGaAsP, and layer L 4  is another InP/InGasP layer with p-dopping (e.g. C).  
         [0032]    Layer L 4  is appropriately patterned, so as to form two parallel spaced-apart linear ridge segments W 1  and W 2 , and two spaced-apart ring-like ridge segments R 1  and R 2 . A metal layer L 5  (the so-called p-metal) (e.g., Ti/Pt/Au) is then deposited on the surface of the patterned p-type semiconductor layer L 4 , and etched to provide metal coating on top of each of the ridge segments. The metal layer can be deposited and etched prior to etching the layer L 4  and may serve also as etch masks for the etching of layer L 4 .  
         [0033]    The linear ridge segments W 1  and W 2  serve as waveguides, and the ring ridge segments R 1  and R 2  serve as resonator loop cavities between the two waveguides, all the waveguides containing an active lasing material. The resonator cavities (rings) serve as frequency-selective mirrors, the resonator cavities and the waveguide sections creating together a closed loop compound resonator. The physical characteristics of the compound resonator can be controlled to adjust its optical storage characteristics (refractive indices). Generally speaking, the change in the refractive index will induce the required phase shift to change the frequency response of the compound resonator. Such an active phase affecting may be achieved by applying any suitable thermo-optic, electro-optic, piezo-electric or the like effects mainly within the ring resonator regions but also on the linear waveguide region between the rings.  
         [0034]    [0034]FIG. 2 illustrates a structure  12  obtained by applying further deposition and etching processes to the structure  10 . As shown, a passivation or burying layer L 6  (e.g., polyimid) is deposited on top of the structure  10 , and is then selectively etched to open contact windows CW, which thus become exposed to further processing.  
         [0035]    [0035]FIG. 3 illustrates the entire laser structure  14  obtained by applying a metal deposition process to the structure  12  to fabricate electrodes E 1 -E 4  above the ridge segments. Thus, electrodes E 1  and E 2 , and electrodes E 3  and E 4  present contact pads for the two linear segments (waveguides) W 1  and W 2  and two ring segments R 1  and R 2 , respectively.  
         [0036]    Turning now to FIG. 4, there is illustrated a cross-sectional view of the structure  14  along line A-A (FIG. 3) showing a light-coupling scheme (arrows  16 ) in the structure  14 . Light is coupled from the waveguide W 1  to waveguide W 2  through the resonator rings R 1  and R 2  in accordance with the resonance condition thereof (selective frequency range). As shown, the electrodes are arranged such that each of the segments can be manipulated (driven) separately by the injection of a current or by applying external fields.  
         [0037]    In the above example, the segments are produced by dry etching of the semiconductor layer structure, and the laser structure of the index-guide type. Reference is now made to FIGS. 5A and 5B, illustrating a laser structure  100  according to another example of the invention, namely, a laser of die gain-guide type.  
         [0038]    [0038]FIG. 5A illustrates a top view of the structure  100 , and FIG. 5B illustrates a cross section thereof taken along line B-B. The structure  100  is constructed generally similar to the previously described example, namely, utilizes the principles of integrated technology for growing the laser layer-structure L 1  (n-type layer L 2 , active laser layer L 3  and p-type layer L 4 ) on the substrate layer L 0 . The same materials and relative disposition of layers as described above with respect to the device  10  can be used in the device  100 . Here, however, the waveguide segments W 1 , W 2 , R 1  and R 2  are produced by a planar process based on proton implantation within the areas surrounding the segments.  
         [0039]    Thus, proton-implanted areas  18  are formed in the p-semiconductor layer L 4  within spaces between the ridge segments, thereby generating insulation between the ridge segments. A further insulating layer L 7  (SiO 2  or Si 3 N 4  or polyImid is then deposited and patterned, or deposited through a mask, in a manner to enable the appropriate fabrication of contact pads (electrodes) E 1 -E 4 , such that each of the segments can be manipulated separately (by injection of a current or by applying electrical, thermal or mechanical fields).  
         [0040]    [0040]FIG. 6 illustrates a laser diode structure  200 , differing from the above-described examples in that only two linear waveguide segments contain the laser active material. The structure  200  comprising the laser layer-structure L 1  is grown on a substrate layer Lo in the above-described manner. Linear waveguides W 1  and W 2  are defined by conventional dry etching, and a deep etch trend (underneath at least layer L 3 —the active layer) is then etched between the two linear ridge segments defining an interlaser spacing S. The interlaser spacing S is subsequently coated by a thin buffer layer L 8  made of low refraction index material (e.g., SiO 2 ), and a layer L 9  of a higher index material (e.g., Si 3 N 4 , Si). Ring segments R 1  and R 2  are defined in the high index material layer L 9 . The linear and ring segments are buried by depositing a passivation layer L 6 . The ring segments R 1  and R 2  can be used to tune the laser, for example, by a thermooptic effect.  
         [0041]    [0041]FIGS. 7A and 7B present structures according to two more examples of the invention, respectively, illustrating how the laser structure according to the invention can, be coupled to other functional devices. In the example of FIG. 7A, a structure  300  utilizes a flared amplifier  20  as a functional device coupled to the laser structure. The operation of the flare amplifier is known per se and therefore need not be specifically described, except to note the following. In the structure  300 , due to the provision of ring resonators R 1  and R 2  acting as mirrors, the need for a DBR type or cleaved mirror between the laser and the flared amplifier (which is typically provided in the conventional devices of the kind specified and which is very difficult to manufacture) is eliminated. In a structure  400  of FIG. 7B, such functional devices to which the laser structure according to the invention can be easily coupled are the flared amplifier  20  and an additional waveguide W 3 , which may serve as a part (active element) of another integrated optical device or be a passive waveguide directing light to another device(s).  
         [0042]    In the above examples of FIGS. 7A and 7B, the linear waveguide segments are coupled to external segments (functional devices) horizontally. It should, however, be understood that vertical connection between these segments through. one or two rings could be provided, thereby providing even more miniature integrated optical device.  
         [0043]    Those skilled in the art will readily appreciate that various modifications and changes can be applied to the preferred embodiments of the invention as hereinbefore exemplified without departing from its scope defined in and by the appended claims.