Abstract:
An electroabsorption modulated laser (EML) is formed to include a Fabry-Perot lasing section, in place of the conventional DFB lasing section. When operated at a wavelength of 1310 nm, the wider spectral bandwidth of the FP device (containing several longitudinal modes) is of no concern, since 1310 nm is the zero dispersion wavelength of most conventional transmission fibers. A selective area growth process is used to simultaneously form the MQW active regions of both the FP and EA sections of the EML device, and an isolation trench may be formed between the sections to reduce the effects of electrical crosstalk.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of Provisional Application No. 60/218,918, filed Jul. 18, 2000. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an electroabsorption-modulated laser (EML) device and, more particularly, to an EML operating at a wavelength of 1310 nm by using a Fabry-Perot (FP) structure as its lasing section. 
     TECHNICAL FIELD 
     Optical transmitters, transceivers, and transponders utilizing electroabsorption modulated lasers (EMLs) at bit rates of up to 10 Gb/sec are of increasing interest, particularly when compared to the performance of directly modulated lasers. The zero dispersion property of conventional optical fiber at wavelengths close to 1300 nm makes the option of using a 1300 nm EML even more attractive than a 1550 nm EML, due to the non-existent dispersion penalty at the latter wavelength. Further, the modulation speed of an EML device is limited only by the RC time constant in the modulator section of the device. EML devices with a small enough device capacitance enable system designers to realize transmitters and transponders with speeds over 10 Gb/sec without the complication of the relaxation resonance frequency effects incurred in the directly modulated distributed feedback (DFB) or Fabry Perot (FP) lasers. In additional EML devices can be designed with a very high extinction ratio and system sensitivity penalties associated with this parameter can be virtually eliminated. 
     At the present invention, 1500 nm EMLs are being deployed in high speed (e.g., 2.5 Gb/sec and 10 Gb/sec) fiber optic networks. One exemplary EML arrangement is discussed in the article The advantage of these devices, as compared to conventional directly modulated DFBs, is that EML lasers exhibit highly superior eye diagrams, with less pulse distortion/ringing minimal chirp characteristics, high extinction ratio, and simplified driver circuitry. However, these advantages are obtained at the expense of a more complicated chip design, requiring multiple epitaxial regrowth steps, and additional wafer processing procedures to obtain electrical isolation between the electroabsorption modulator and laser sections of the device. Moreover, EML devices require tightly confined matching of the optical characteristics between the DFB grating, the active material gain characteristics, and the modulator material absorption characteristics. The combination of these factors can result in low yields and high costs, particularly when manufactured in high volume. Therefore, any new innovations which can relax any of the above requirements of the EML design can have considerable commercial impact and lead to competitive advantages. 
     At the same time, there is a rapid increase in the deployment of fiber-optic-based equipment which utilizes transponder, transceiver and transmitter modules operating at 10 Gbit/sec and at wavelengths near the 1310 nm dispersion minimum of the optical fiber. Currently, directly modulated 1310 nm DFB or FP lasers are utilized in these types of applications. However, directly modulated DFB or FP lasers exhibit severe limitations due to relaxation oscillation effects and the difficulties of modulating the drive current at 10 Gb/sec. The advantages of utilizing an EML laser operating at 1310 nm are significant, for the same reasons outlined in the previous paragraph. Some development work has moved forward regarding a 1310 nm EML, as discussed in the article “10 Gbit/s penalty-free transmission over 48 km using 1.3-μm wavelength electroabsorption modulated lasers (EML) for metro-loop transmission link”, by T. Tanbun-Ek et al, appearing in OFC Proceedings, San Jose Calif., 1996, at p.207. However, the research discussed in this and other papers presumes the use of a DFB section as the lasing device, requiring a complicated fabrication sequence, which is subject to low yields and high costs. 
     Thus, a need remains in the art for an EML device that is less costly and easier to manufacture than DFB-EML devices, yet is capable of operating at a wavelength of approximately 1310 nm. 
     SUMMARY OF THE INVENTION 
     The need remaining in the prior art is addressed by the present invention, which relates to an electroabsorption-modulated laser (EML) device and, more particularly, to an EML operating at a wavelength of 1310 nm by using a Fabry-Perot (FP) structure as its lasing section. 
     In accordance with the present invention, an FP laser and electroabsorption (EA) modulator are simultaneously formed on a common InP substrate, using (for example) a selective area growth (SAG) process. A trench is formed between the top conducting layers of the two sections to provide for electrical isolation between the FP laser and the EA modulator. 
     It is an advantage of the present invention that even though an FP laser exhibits a wider spectral bandwidth than the DFB laser conventionally used in an EML structure, the bandwidth is not a concern when operating at 1310 nm, the zero dispersion wavelength of the transmission fiber. 
     Other and further advantages of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the drawings, where like numerals reference like parts in several views: 
     FIG. 1 contains an isometric view of a Fabry-Perot electroabsorption modulated laser (FP-EML) formed in accordance with the present invention; 
     FIG. 2 is an end view of the structure of FIG. 1; and 
     FIGS. 3-7 illustrate an exemplary set of process steps used to form the FP-EML of the present invention. 
    
    
     DETAILED DESCRIPTION 
     A cut-away isometric view of a Fabry-Perot electroabsorption modulated laser (FP-EML)  10  formed in accordance with the present invention is illustrated in FIG. 1, comprising an FP laser section  12  and EA modulator section  14 . FIG. 2 contains an end view of FP-EML  10 , illustrating the laser end facet of the device. As in the case of all electroabsorption modulated devices, FP laser  12  section is operated in CW mode and EA modulator section  14  is subjected to an input electrical modulation signal to cause the optical output modulation in laser section  12 . Both FP laser  12  and EA modulator  14  are formed on a common InP substrate  16 , where FP laser  10  comprises a multiple quantum well (MQW) action region  18  which transitions to become a MQW action region  20  in EA modulator  18 . As will be described in detail below in association with FIGS. 3-5, a selective area growth (SAG) technique may be used to form this MQW structure and insure that active region  18  in laser  12  is emissive (relatively thick MQW layers), while active region  20  in EA modulator  14  is absorptive (relatively thin MQW layers). The transition between the MQW layer thickness is evident in area  22  of the cut-away view of FIG.  1 . 
     Referring to both FIGS. 1 and 2, FP-EML  10  comprises a first n-InP buffer layer  22 , covered by a separate confinement heterostructure (SCH) InGaAsP layer  24 . Preferably, first n-InP buffer layer  22  comprises a thickness on the order of 100 nm and SCH layer  24  comprises a thickness of approximately 70 nm and exhibits a band gap wavelength of 1.15 micron. MQW action regions  18  and  20  are formed over SCH layer  24 , preferably using the SAG process. In a preferred embodiment, between 7 and 9 pairs of “barrier”  26  and “well”  28  layers are formed, where for FP laser section  12 , the layers are grown in a manner to provide lasing at the desired wavelength. For some embodiments, a device which lases in the wavelength range of 1260-1600 nm is desirable. Other arrangements require a device which lases in the wavelength range of 700-1000 nm. Some conventional EML devices have exhibited excellent characteristics at a wavelength of approximately 1550. An advantage of the FP-EML structure of the present invention is that the FP device can be formed to exhibit a wavelength of 1310 nm, which cannot be achieved using a conventional DFB device in the EML structure. In general, the use of a SAG process to form the MQW active region allows for the FP-EML device of the present invention to be tailored to emit at a wavelength chosen by the designer for a specific system implementation. 
     Referring back to FIG. 1, a second InGaAsP SCH layer  30  is formed over MWQ active regions  18 ,  20 . Current blocking in the device is provided by Fe-doped InP barriers  32  and  34 , formed on either side of the active waveguiding region of FP-EML  10 . 
     A p-InP cladding layer  36  is then formed on the top surface of device  10 , followed by a p-InGaAs contact layer  38 . A first electrical contact pad  40 , associated with FP laser  12 , is deposited on contact layer  38  over the location of active region  18 . A second electrical contact pad  42 , associated with EA modulator  14 , is deposited on contact layer  38  over the location of active region  20 . In a preferred embodiment, first and second electrical contact pads comprise a tri-layer Ti—Pt—Au structure. 
     In accordance with the properties of the FP-EML device of the present invention, an isolation trench  44  is formed between FP laser section  12  and EA modulator  14 , as shown in FIG.  1 . In a preferred embodiment, trench  44  comprises a depth of approximately 0.7 microns (into p-InP cladding layer  36 ) and a width of approximately 20 microns. Trench  44  may be formed using conventional reactive ion etching (RIE) techniques and is used to reduce electrical crosstalk between FP laser section  12  and EA modulator section  14 . 
     FIGS. 3-7 describe an exemplary set of processing steps for fabricating an FP-EML device in accordance with the present invention. Referring to FIG. 3, InP substrate  16  is first covered with a relatively thick (e.g., 300 nm) layer  50  of SiO 2 , which may be deposited using a convention PECVD technique. Layer  50  is then masked and etched to form a pair of suitable width stripes  52  and  54  (for example, 20 μm in width), separated by a 20 μm wide barrier  56 . A separation FP-EML device is formed, in accordance with the present invention along each stripe  52  and  54 , with barrier  56  used maintain separation between the devices. Once fabrication is completed, the structure is cleaved along the dotted line shown through barrier  56  to separate the devices. It is to be understood that the use of 20 μm as the width for both the stripes and the barrier is a matter of design choice, and has been found to provide the necessary transverse guiding of the output from FP laser section  12 . FIGS. 4-7 depict the formation of active regions  18  and  22 , starting with the deposit of n-InP buffer layer  22  using an MOCVD process. For the remainder of the discussion, the fabrication steps will illustrate the formation of only a single device, not a pair of devices as shown in FIG.  3 . As mentioned above, buffer layer  22  may be formed to comprise a thickness of approximately 100 nm. While remaining in the MOCVD reactor, InGaAsP SCH layer  24  is next deposited, to a thickness of approximately 70 nm. A selective area growth (SAG) process in then initiated (still in the MOCVD reactor), to form the MQW structure active regions  18  and  20  of FP laser section  12  and EA modulator section  14 , respectively. FIG. 5 illustrates the formation of active region  18  of FP laser section  12  and FIG. 6 illustrates the formation of action region  20  of EA modulator section  14 . As is well-known in the art, a “multiple quantum well” structure comprises alternating layers having a relatively small energy gap (“well layers”, i.e., layer  28 ) and a relatively large energy gap (“barrier” layers, i.e., layer  26 ). In accordance with the present invention, layers  26  and  28  in FP active region  18  are thicker than in EA active region  20 . Only this difference in layer thickness makes it possible to operate section  12  as an active laser and section  14  as an EA modulator. Using the SAG process, surface masking processes are used to impede the growth of each of the barrier and well layers in EA modulator section  14  after a predetermined period of time, allow the barrier and well layers in FP laser section  12  to be relatively thicker. The result is as shown in FIGS. 5 and 6 (where the illustrated difference in the MQW structures is exaggerated for the sake of clarity). Subsequent to the SAG process of forming the MQW structure, an upper SCH p-InP layer  30  is formed to cover active regions  18 , 20 , using the same MOCVD process. 
     Once the active areas of FP laser section  12  and EA modulator  14  are completed, an epitaxial growth process is performed, using Fe-doped InP, to form current blocking regions  32  and  34  on either side of active regions  18 , 20 , as shown in FIG.  7 . The use of Fe-doped InP will impede the spread of the current beyond the central channel region and will also control the transverse mode of the wave propagating through the active region of the device structure. P-InP cladding layer  36  and p-InGaAs contact layer  38  are then grown, in blanket form, to complete the structure. Once these layers are in place, trench  44  (see FIG. 1) may be formed using an RIE process, and contacts  40 ,  42  deposited at the proper locations on the surface of contact layer  38 . 
     If formed in pairs, the final structure is cleaved along the dotted line shown in FIG.  3 . The prior high reflectivity (e.g., 99%) (HR) and anti-reflection (e.g., 10%) (AR) coatings  46 ,  48  are then formed on the facets of FP laser device  12  and EA modulator device  14 , respectively.