Abstract:
An optical bandwidth source for generating amplified spontaneous emission (ASE) across a selected wavelength range, the optical bandwidth source including a waveguide having a first end and a second end, and comprising a plurality of separate wavelength gain subsections arranged in a serial configuration between the first end and the second end so as to collectively form an active waveguide between the first end and the second end; wherein each of the wavelength gain subsections is configured to produce ASE across a wavelength range which is less than, but contained within, the selected wavelength range, whereby the plurality of separate wavelength gain subsections collectively produce ASE across the selected wavelength range.

Description:
REFERENCE TO PENDING PRIOR PATENT APPLICATIONS  
       [0001]     This patent application:  
         [0002]     (1) is a continuation-in-part of pending prior U.S. patent application Ser. No. 10/632,779, filed Aug. 1, 2003 by Daryoosh Vakhshoori et al. for SYSTEM FOR AMPLIFYING OPTICAL SIGNALS (Attorney&#39;s Docket No. AHURA-1);  
         [0003]     (2) claims benefit of pending prior U.S. Provisional Patent Application Ser. No. 60/454,036, filed Mar. 12, 2003 by Daryoosh Vakhshoori et al. for EXTENDED OPTICAL BANDWIDTH SEMICONDUCTOR SOURCE (Attorney&#39;s Docket No. AHURA-5 PROV); and  
         [0004]     (3) claims benefit of pending prior U.S. Provisional Patent Application Ser. No. 60/549,310, filed Mar. 2, 2004 by Kevin J. Knopp et al. for BANDWIDTH ADJUSTABLE BROADBAND LIGHT SOURCE FOR OPTICAL COHERENCE TOMOGRAPHY (Attorney&#39;s Docket No. AHURA-20 PROV).  
         [0005]     The three (3) above-identified patent applications are hereby incorporated herein by reference.  
     
    
     FIELD OF THE INVENTION  
       [0006]     This invention relates to optical components in general, and more particularly to optical components for generating light.  
       BACKGROUND OF THE INVENTION  
       [0007]     In many applications it may be necessary and/or desirable to generate light.  
         [0008]     Different optical components are well known in the art for generating light. By way of example but not limitation, semiconductor lasers, such as vertical cavity surface emitting lasers (VCSEL&#39;s), are well known in the art for generating light. Depending on the particular construction used, the light source may emit light across different portions of the wavelength spectrum. By way of example, many semiconductor-based light sources emit light across a relatively narrow portion of the wavelength spectrum. However, in many applications it may be necessary and/or desirable to provide a semiconductor light source which emits light across a relatively broad band of wavelengths.  
         [0009]     The present invention is directed to a novel semiconductor light source for emitting light across an extended optical bandwidth.  
       SUMMARY OF THE INVENTION  
       [0010]     The present invention provides an optical bandwidth source for generating amplified spontaneous emission (ASE) across a particular wavelength range, the optical bandwidth source comprising:  
         [0011]     a waveguide having a first end and a second end, and the waveguide having a plurality of separate wavelength gain subsections arranged in a serial configuration to form an active waveguide between the first end and the second end;  
         [0012]     wherein each of the wavelength gain subsections is arranged relative to one another so as to produce ASE across the particular wavelength range.  
         [0013]     In another form of the invention, there is provided a system for generating amplified spontaneous emission (ASE) across a particular wavelength range, the system comprising: 
        an optical bandwidth source for generating the ASE across the particular wavelength range, the optical bandwidth source comprising: 
            a waveguide having a first end and a second end, and the waveguide having a plurality of separate wavelength gain subsections arranged in a serial configuration between the first end and the second end;     wherein each of the wavelength gain subsections is arranged relative to one another so as to produce ASE across the particular wavelength range;     a thin-film tap configured adjacent to the second end of the waveguide to divert a portion of the ASE produced by the waveguide to an auxiliary pathway;     a power monitor configured to receive the portion of the ASE diverted along the auxiliary pathway so as to monitor the ASE produced by the optical bandwidth source;     an isolator configured to receive the ASE remaining from the portion diverted toward the power monitor, the isolator configured to eliminate feedback therethrough toward the waveguide; and     a single-mode filter fiber pigtail configured adjacent to the isolator in opposition to the waveguide so as to receive ASE emitted from the waveguide after passing through the isolator.    
               
 
         [0021]     In another form of the invention, there is provided a method for generating amplified spontaneous emission (ASE) across a particular wavelength range, the method comprising:  
         [0022]     providing a waveguide having a first end and a second end, and the waveguide having a plurality of separate waveguide gain subsections arranged in a serial configuration to form an active waveguide between the first end and the second end; and  
         [0023]     electrically biasing a first waveguide gain subsection and a second waveguide gain subsection from the plurality of separate waveguide gain subsections, the first waveguide gain subsection being configured between the first end and the second waveguide gain subsection, the second waveguide gain subsection being configured between the second end and the first waveguide gain subsection, and the first waveguide gain subsection configured with a quantum-well structure having a bandgap with lower energy than the second waveguide gain subsection so as to produce longer wavelength ASE at the first waveguide gain subsection than at the second waveguide gain subsection, wherein the waveguide produces ASE across the particular wavelength range at the second end thereof formed by ASE produced by the first waveguide section and the second waveguide section. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]     These and other objects and features of the present invention will be more fully disclosed or rendered obvious by the following detailed description of the preferred embodiments of the invention, which is to be considered together with the accompanying drawings wherein like numbers refer to like parts and further wherein:  
         [0025]      FIG. 1  is a schematic view illustrating one preferred form of broadband semiconductor light source formed in accordance with the present invention;  
         [0026]      FIG. 2  is a schematic view illustrating one preferred form of broadband semiconductor light source module formed in accordance with the present invention;  
         [0027]      FIG. 3A  is a schematic top view of a package incorporating the broadband semiconductor light source module shown in  FIG. 2 ;  
         [0028]      FIG. 3B  is a schematic side view of a package incorporating the broadband semiconductor light source module shown in  FIG. 2 ; and  
         [0029]      FIG. 3C  is a schematic end view of a package incorporating the broadband semiconductor light source module shown in  FIG. 2 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     Overview  
       [0030]     The present invention is based on a novel seeded power-optical-amplifier (SPOA) technology. This technology relies on the amplification of a low-power seed optical spectrum by a long-cavity semiconductor waveguide optimized for power amplification. This SPOA technology results in a high-power (&gt;200 mW) broad-band (˜35 nm) source available from 650 to 1650 nm. To address market needs for sources of lower-power with an extended spectral bandwidth of 100 to 200 nm, the SPOA sources are serially-multiplexed. This approach addresses markets such as optical coherence tomography and spectral-sliced wavelength division multiplexing.  
         [0031]     This novel broadband semiconductor light source provides significant advantages in performance, size, and cost over traditional semiconductor and super-continuum light sources.  
         [0032]     Some of the technical advantages of this novel platform are:  
         [0033]     (i) extended spectral bandwidth (FWHM of 100 to 200 nm);  
         [0034]     (ii) high power (&gt;20 mW);  
         [0035]     (iii) single integrated chip: no spectral “stitching” or external combining is required;  
         [0036]     (iv) smooth spectral shape with low secondary coherence function;  
         [0037]     (v) compact size and low electrical power consumption; and  
         [0038]     (vi) compatible with reliable telcom-qualified packaging techniques.  
       Serial-Multiplexed Seeded Power-Optical-Amplifier  
       [0039]     A schematic representation of a novel serial-multiplexed, seeded power-optical-amplifier (SM-SPOA) broadband light source die  5  is shown in  FIG. 1 .  
         [0040]     The device  5  consists of a curved active waveguide  10  having a plurality of gain, or seed, subsections  12  serially disposed along waveguide  10 . Preferably waveguide  10  is a single mode waveguide, although it may also be a multi-mode waveguide. Each gain, or seed, subsection  12  is adapted to generate amplified spontaneous emission (ASE). The gain profile along the waveguide  10  is engineered to generate ASE across a broad wavelength range (100-200 nm) when electrically biased above transparency. This is accomplished by varying the bandgap of the gain from lower to higher energy along the length of the waveguide in a discrete or continuous fashion using techniques such as semiconductor regrowth or quantum-well intermixing. In one preferred construction, each gain subsection  12  is configured to generate a different ASE profile. In another construction, waveguide  10  may be configured to have a continuous gradation along its length to change the bandgap and thus present what is essentially an infinite number of subsections  12 . The ASE generated from the lower energy segments of the waveguide passes through the higher energy portions with low optical loss (&lt;2 cm−1). An angled waveguide (8-13 degrees) is used at the output of the device, followed by an antireflection coating  15  deposited on the semiconductor facet. This combination is used to reduce feedback (&lt;-50 dB) into the device and thus prevent distortion of the broadband spectral profile from Fabry Perot interference. The output will be highly linearly polarized due to the polarization dependence of the quantum-well gain. A high reflecting coating  17  is preferably placed on the opposite end of the device, e.g., at the end adjacent the low energy end of the waveguide.  
       Gain Subsections  
       [0041]     The basic principle of device operation is the amplification of a plurality of gain, or seed, spectrums of amplified spontaneous emission (ASE) along the length of a semiconductor waveguide containing active regions which are biased above transparency. The manner in which the seed light is generated and shaped (i.e., filtered), the number of gain, or seed, spectrums used, and the optical bandgap and electrical bias of those sections, all may be varied according to the particular design considerations to be addressed. The semiconductor material system used depends to a large extent on the wavelength of the desired application. Among others, material systems such as AlAs, GaAs, InP, GaP, InGaAs, InGaAsP, InAlGaAs, and GaN can be used.  
         [0042]     The die  5  consists of a serial connection of multiple gain subsections  12  formed along the semiconductor waveguide  10 . Nine gain subsections  12  are shown in  FIG. 1 ; however, it should be appreciated that this number is merely exemplary and more or less than this number of wavelength gain subsections may be used. The gain profile within each gain subsection  12  is chosen so as to provide ASE in a particular wavelength range. The gain profiles can be defined in each gain subsection  12  by such techniques as epitaxial regrowth or quantum-well intermixing. The quantum-well blocks of these gain subsections are designed to provide a region of high gain with, for example, 3-10 quantum wells.  
         [0043]     A high reflectance mirror  17  is used to capture and redirect the portion of seed light traveling away from the output end of the device. The spectral profile of this mirror  17  is designed to provide the desired nominal ASE spectrum. This high reflectance mirror  17  can be defined through thin film coating of the cleaved semiconductor facet or by incorporating a distributed Bragg reflector along the waveguide. Each wavelength gain subsection  12  has an independent electrical contact to allow dynamic tailoring of the seed light spectrum. The output power of the wavelength gain sections  12  can range from 1 to 20 mW, although it is not limited to this range.  
         [0044]     An angled waveguide  10  is used at the output of the device, followed by an antireflection coating  15  on the semiconductor facet. This combination is used to eliminate feedback into the device and to prevent distortion of the broadband spectral profile from Fabry-Perot interference.  
         [0045]     The output of the device will be highly linearly polarized because of the polarization dependence of the quantum-well gain or, in the case of bulk active region, excess loss of TM over TE mode.  
         [0046]     The spectral shape of the ASE generated by the device can be dynamically varied by changing the electrical bias applied to the various gain sections  12 .  
       Optoelectronic Packaging  
       [0047]     Looking next at  FIG. 2 , the semiconductor die  5  may be soldered to an aluminum nitride carrier  20  and be packaged with its associated optical components so as to form a module  23 . A thin-film tap  25  and photodetector  30  may be included to provide power monitoring functionality. The thin-film tap  25  is preferably also used for spectral shaping. More particularly, the thin-film coating on this optic is preferably designed to not only reflect a small fraction of light (e.g., 1%) to an auxiliary path, but also to refine and further shape the optical spectrum emitted from the semiconductor device. For many applications, features such as spectral ripples must be removed. The thin film coating preferably helps to do this and adjust the spectrum to approach the ideal Gaussian shape. Also, if desired, this optic could be stand-alone as a separate element from the tap and/or dynamically configurable. An optical isolator 35 may be used to eliminate feedback from downstream in the system. A thermoelectric cooler (TEC) (not shown, but preferably provided beneath aluminum nitride carrier  20 ) may be used to maintain the temperature of the entire optical platform. The optical train may be contained in a 14-pin hermetically-sealed butterfly package  40  with a single-mode fiber pigtail  45 .  
         [0048]      FIGS. 3A, 3B  and  3 C show further details of the optical module  23  is shown in  FIG. 3 .  
       Optical Performance Specifications  
       [0049]     In a preferred embodiment of the present invention, the broadband source module provides the performance criteria outlined in Table 1 over its life throughout the environmental conditions specified in Table 4. The specifications for the final product, alpha prototypes, and beta units are listed; however, it should be appreciated that this table is provided by way of example only and not by way of limitation.  
                                                                             TABLE 1                                                   Final       Parameter   Unit   Min   Typical   Max   α   β   Product                                Output Optical Power   mW   10   25       √   √   √       Spectral Bandwidth   nm   100           √   √   √               200       Center Wavelength   nm   1290   1300   1310   √   √   √       Secondary Coherence Lobe 3     dB   30   50       √   √   √       Relative Intensity Noise   dB/Hz           −100       √   √                       f &lt; 1 GHz                  
 
       Mechanical Assembly  
       [0050]     In a preferred embodiment of the present invention, there is provided a broadband source module having the mechanical attributes specified in Table 2 for the final product, alpha prototypes, and beta units; however, it should be appreciated that this table is provided by way of example only and not by way of limitation.  
                                   TABLE 2                                           Final       Parameter   Unit   Value   α   β   Product                   Fiber Type   Type   Single-Mode   √   √   √       Fiber Connector   Type   Bare   √   √   √       Fiber Pigtail Length   m   &gt;1   √   √   √       Package Style of Optical   Type   14-Pin Butterfly   √   √   √       Module       Dimensions of Optical   mm   42 × 12 × 13       √   √       Module       Sealing of Optical Module   Type   Hennetic       √   √                  
 
       Electrical Specifications  
       [0051]     In a preferred embodiment of the present, a laser source module has the electrical requirements specified in Table 3 for the final product, alpha prototypes, and beta units; however, it should be appreciated that this table is provided by way of example only and not by way of limitation.  
                                                                             TABLE 3                                                   Final       Parameter   Unit   Min   Typical   Max   α   β   Product                                SM-SPOA Current   V   0   2   2.3       √   √       Driver   A   0   0.5   1.5       √   √       TEC Driver   V   −1.5       1.5       √   √           A   −1.5       1.5       √   √       Power Dissipation 4     W           5       √   √       Thermistor   kΩ   9.5   10   10.5   √   √   √       Resistance       (@ 25° C.)       Monitor Photodiode   nA           100       √   √       Dark Current       (V reverse  = 5 V)       Signal Power   μA/mW   3.8   4   4.2       √   √       Monitor       Responsivity       (V reverse  = 5 V)                  
 
       Environmental Conditions  
       [0052]     The environmental operating conditions are shown in Table 4; however, it should be appreciated that this table is provided by way of example only and not by way of limitation.  
                                   TABLE 4                                           Final       Parameter   Unit   Value   α   β   Product                   Operating Temperature   ° C.   5 to 45       √   √       Storage Temperature Range   ° C.   −40 to 75        √   √       Operating Humidity Range   %   0 to 90       √   √                  
 
       Qualification  
       [0053]     The broadband source module has a mean time to failure (MTTF) of greater than 10,000 hours. End of life (EOL) is considered to occur when the specifications of Table 1 can no longer be met. Processes and techniques compatible with Telcordia qualification standards are preferably used to ensure reliable operation. Qualification testing includes: aging, storage, damp-heat, thermal cycling, and mechanical shock/vibration. Other tests may be performed as needed to ensure product quality.