Abstract:
An optical transmitter providing the benefits of both filter-locked and wavelength-locked lasers is disclosed by modifying an external cavity ( 32 ) for the integration of an optical modulator ( 14 ). The external cavity ( 32 ) provides a round-trip path for light travel. A substrate ( 24 ) is connected to the external cavity ( 32 ) where at least one gain element ( 16 ) and the optical modulator ( 14 ) are integral with the substrate ( 24 ). A partial reflector ( 40 ) is also integral with the substrate ( 24 ) and couples the at least one gain element ( 16 ) with the optical modulator ( 14 ).

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to lasers, and particularly to external cavity lasers for use as a transmitter in optical communications. 
     2. Technical Background 
     In dense wavelength division multiplexing (DWDM) system applications, the transmitter wavelength has to be locked to one of the International Telephone Union (ITU) standard wavelengths of an ITU grid to meet crosstalk specification and ensure reliable operation of the system over its lifetime (about 25 years). The lasing wavelength of a free running commercial distributed feedback (DFB) laser, determined by its built-in DFB grating and refractive index of the semiconductor waveguide, changes with temperature at a rate of 0.1 nm/° C. FIG. 11 shows a wavelength-locked DFB laser demonstrated by Nortel Technology as described in the article by B. Villeneuve, H. B. Kim, M. Cyr and D. Gariepy, “A compact wavelength stabilization scheme for telecommunication transmitter,”  Digest of the LEOS Summer Topical Meetings, WDM Components Technology , WD2, 19-20, Aug. 13-15, 1997, Montreal, Quebec, Canada. A slightly diverging beam of laser light  112  transmitted through a Fabry Perot filter or a single-cavity multilayer dielectric filter  114  is detected by two closely spaced photodetectors  116  acting as apertures. The photodetectors  116  are equally spaced from the centerline of a semiconductor source laser  118 . Each photodetector  116  captures a different but overlapping center portion of the total solid angle emitted by the divergent laser light source, as the filter  114  is aligned to control and monitor the transmission wavelengths. Two different spectral responses, offset in wavelengths according to their angular difference, are produced as shown in FIG.  12 . The difference or discrimination signal  222  is used by an operational amplifier  220  to control a heat sink temperature to lock the lasing wavelength to the ITU wavelength or center frequency λ 0 . 
     FIG. 12 shows the ideal case where the wavelength offset between the two responses is roughly equal to their effective bandwidths such that the center frequency is centered at the ITU wavelength. However, to reduce cost, it is desirable to eliminate the extra external feedback parts of the operational amplifier  220  and photodiodes  116  needed for wavelength discrimination in this type of temperature control of a wavelenth-locked laser while maintaining or improving temperature reliability. 
     Filter-locked external cavity lasers as shown in FIGS. 13 and 14 have recently been proposed and demonstrated. These lasers do not need the feedback control to monitor wavelengths since the center wavelength of its filters, made of a dielectric material, such as the fiber grating and the multilayer dielectric filter, has been demonstrated to be much less sensitive to temperature (&lt;0.005 nm/° C.) than that of the semiconductor grating filter used in the DFB laser. A reflective Bragg grating written into the fiber establishes the precise lasing wavelength. One of the frequencies of the ITU grid is selected for the Bragg grating. The advantage of writing the frequency into the silica fiber is that the silica has a small coefficient of thermal expansion (about 5×10 −7 /° C.) and the resonant Bragg frequency changes can be made negligible by temperature compensation. 
     As seen in FIG.  13  and described in U.S. Pat. No. 5,844,926, a semiconductor laser diode chip  118  is provided with an anti-reflection (AR) coating  26  on one end facet  132  to which is optically coupled a length of optical fiber pigtail  134  in which there is a Bragg grating reflector  136  defining one end of a laser optical cavity whose other end is provided by the reflecting end facet  138  of the laser chip remote from the AR coated end facet. This Bragg grating reflector thus provide a means of locking the laser frequency. 
     Instead of using fiber, air can be substituted in the external cavity of FIG.  14 . Here and described in U.S. Pat. No. 5,434,874 and U.S. Pat. No. 5,870,417, a gain medium, such as the semiconductor (laser chip)  118  has both front  138  and back  132  facets, as in FIG. 13, where the back facet  132  has the anti-reflection coating  26 . Light  142  from the laser chip passes through the back facet  132  into an external air cavity. The cavity contains a tuning element  162 , such as a prism, mirror, filter, or grating, that reflects specific laser wavelengths back into the laser chip  118 . This round-trip light action  142  causes the laser to output selectable wavelengths  62  through the front facet  138 . Thus, the wavelength of light output  62  from the front  138  facet of the laser chip can be controlled by changing the angle of the grating, filter or other tuning element  162 . The cavity also contains a collimating lens  144  which directs light emitted from the rear facet  132  of the chip onto the grating, filter, or other tuning element  162 . 
     However, due to its long external cavity, the filter-locked laser can not be directly modulated at a high bit rate. The 3-dB modulation bandwidth decreases as the external passive cavity length increases as seen in FIG.  9 . For example, the direct modulation bandwidth of a semiconductor laser with a 300 μm cavity length is about 10 GHz. Therefore, it is difficult to directly modulate a filter-locked laser at a rate of 2.5 Gbit/sec and beyond since the external passive cavity length is in the order of 1 cm or longer. Moreover, the direct modulation response at the frequency (peak frequency) corresponding to the round trip time is significantly enhanced as shown in FIG.  15 . The peak frequency as a function of the external passive cavity length is shown in FIG.  10 . If the peak frequency is close to one of the harmonic frequencies of the signal, the signal will be distorted. An external modulator is thus needed for high speed modulation. Additionally, there is at least a cost saving reason to integrate an external cavity laser with an external modulator. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention is an optical transmitter which provides the benefits of both filter-locked and wavelenth-locked lasers is taught by modifying an external cavity for the integration of an external modulator. The external cavity provides a round-trip path for light travel. A substrate is connected to the external cavity where at least one gain element and an optical modulator are integral with the substrate. A partial reflector is also integral with the substrate and couples the at least one gain element with the optical modulator. 
     In another aspect, the present invention includes a saturable absorber integrated between the gain element and the partial reflector. In a further aspect of the invention, the substrate is a waveguide having a first anti-reflection (AR) coated facet and a second opposed AR coated facet on opposed ends of the waveguide. 
     Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings. 
     It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic depiction of a wavelength-locked external cavity laser with an integrated modulator, in accordance with the present invention; 
     FIG. 2 is a diagrammatic depiction of a wavelength-locked external cavity laser with an integrated modulator, having the partial reflector  40  of FIG. 1, embodied as an etched facet in accordance with the present invention; 
     FIG. 3 is a diagrammatic depiction of a wavelength-locked external cavity laser with an integrated modulator, having the partial reflector  40  of FIG. 1, embodied as a waveguide loop mirror in accordance with the present invention; 
     FIG. 4 is a diagrammatic depiction of a wavelength-locked external cavity laser with an integrated modulator, having the partial reflector  40  of FIG. 1, embodied as a distributed Bragg reflector in accordance with the present invention; 
     FIG. 5 is a diagrammatic depiction of a mode-locked external cavity laser with an integrated modulator and an etched facet, in accordance with the present invention; 
     FIG. 6 is a diagrammatic depiction of a mode-locked external cavity laser with an integrated modulator and a waveguide loop mirror, in accordance with the present invention; 
     FIG. 7 is a schematic drawing of the waveguide loop mirror  404  of FIG. 3; 
     FIG. 8 is a graph of the reflectance and transmittance as a function of the crossing ratio of the 2×2 coupler of the waveguide loop mirror  404  of FIG. 3; 
     FIG. 9 is a graph of the 3-dB modulation bandwidth as a function of the external passive cavity length of the external cavity lasers of FIGS. 13 and 14; 
     FIG. 10 is a graph of the peak frequency as a function of the external passive cavity length of the of the external cavity lasers of FIGS. 13 and 14; 
     FIG. 11 is a schematic drawing of a prior-art wavelength-locked DFB laser; 
     FIG. 12 is the spectral responses from the two photodetectors  116  of FIG. 11 separated in space. 
     FIG. 13 is a diagrammatic depiction of a prior-art filter-locked external cavity laser having fiber as its external cavity. 
     FIG. 14 is a diagrammatic depiction of a prior-art filter-locked external cavity laser having air as its external cavity. 
     FIG. 15 is a graph of the small signal modulation response of the external cavity lasers of FIGS. 13 and 14. 
     FIG. 16 is a chart showing a comparison among prior-art wavelength-locked, prior-art filter-locked, and the wavelength locked laser with an integrated modulator of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. An exemplary embodiment of an optical transmitter of the present invention is shown in FIG. 1, and is designated generally throughout by reference numeral  10 . 
     In accordance with the invention, the present invention of a transmitter includes an optical modulator  14  integrated with a filter-locked external cavity laser  12 . An integrated approach is used to build a wavelength-locked external cavity laser  10  by modifying a basic filter-locked external cavity laser that is conventionally implemented as in FIGS. 13 and 14 for the addition of the inventive integrated modulator. 
     FIG. 16 shows the advantages of achievable high speed modulation without the additional components of active feedback realizable with the integration of the modulator in the filter-locked external cavity laser. In accordance with the teachings of the present invention, a wavelength-locked external laser is such provided with an integrated modulator for high speed DWDM system applications. Its inventive features and advantages are compared with other conventional approaches and summarized in FIG.  16 . The transmitter formed by such a wavelength-locked external laser with an integrated modulator is very attractive to DWDM system integrators since such a transmitter eliminates the expensive items of active feedback control to lock its wavelength to the ITU wavelength and the discrete external modulator for high speed modulation. 
     As embodied herein, and depicted in FIG. 1, referencing similar parts from FIGS. 13 and 14, the external cavity laser  12  includes at least one gain element  16  on a first portion of a semiconductor waveguide substrate or chip  24  having a first anti-reflective (AR) coated facet  26  coupled to an external cavity  32  to provide laser spectral properties, or lasing effects, without additional active feedback. By extending the waveguide substrate  24  to form a second substrate portion  34 , the advantages of the present invention can be realized. As embodied herein, and depicted in FIG. 1, the modulator  14  includes a modulator element, such as an electroabsorption or Mach-Zehnder modulator, grown on the second portion or passive region  34  of the substrate  24 , between a partial reflector  40  and a second anti-reflection (AR) coated facet  56  of the laser output end of the waveguide substrate  24  to provide high speed modulation. The rest of the second portion  34  of the substrate  24  includes the partial reflector  40  that is preferably broadband and the inventive second AR coated facet  56 . 
     The partial reflector  40  is needed to form one end of a modified extended laser cavity, in accordance with the present invention. The other end of the modified extended cavity is provided conventionally by an external reflector or external mirror  11 , preferably in the form of an external wavelength selective filter or other tuning element  162  of FIG. 2, if in an air cavity or by the Bragg grating reflector  136  of FIG. 13, if in a fiber cavity. The partial reflector  40  serves to transmit part of the light as an output to the modulator  14 . Additionally, the partial reflector  40  is preferably broadband with respect to the bandwidth of the external reflector or mirror  1 , such as the external wavelength selective reflector  162  or grating such that the lasing wavelength is solely determined by the Bragg wavelength of the wavelength selective filter  162  or the Bragg grating. 
     In order to modulate light  62  externally on a single chip or substrate  24 , the broadband partial reflector  40  is created, inserted or otherwise interposed between the gain  16  and modulator  14  sections or portions of the waveguide substrate chip  24 . The gain element  16  on the first substrate section or portion- 22 , as part of the external cavity laser  12 , has an active layer with a bandgap wavelength of 1570 nm. If the electroabsorption modulator is utilized, the modulator  14  on the second section  34  has a layer, in the waveguide region, with a bandgap wavelength of 1490 nm. The light is modulated by the electroabsorption effect through the electroabsorption modulator  14 . Under zero bias, the light goes through the modulator  14  with little attenuation. Under reverse bias, the light is absorbed since the bandgap of the modulator  14  is red shifted. The laser  16  output is taken from the modulator side or end of the waveguide  24 . Anti-reflection (AR) coatings are preferably applied on both end facets  26  and  56  to avoid spectral degradation due to potential compound cavity effects. The fabrication process to form these two substrate sections or portions  22  and  34  has been used to create the modulator-integrated DFB lasers as described in the article “Performance study of a 10-wavelength DFB laser array with integrated electroabsorption modulators,”  Proc. of IEEE Lasers and Electro-Optics Society Annual Meeting , ThI2, Boston, Mass., Nov. 18-21, 1996. 
     FIGS. 2 and 3 show at least two approaches out of various embodiments for the partial broadband reflector  40 . The broadband internal partial reflector can be formed by either an etched facet (FIG. 2) or a waveguide loop mirror (FIG.  3 ). Although the etched facet of FIG. 2 is very compact, its fabrication is intricate. A good dry etch process is used to form a suitable mirror and accurate dimension control is applied to achieve low insertion loss and provide an accurate splitting ratio. 
     In an alternate embodiment of the invention, as embodied herein and as shown in FIG. 3, the internal reflector  40  of FIG. 1 is a waveguide loop mirror  404 . The reflectance and transmittance of a waveguide loop mirror are controlled by the splitting ratio of the 2×2 coupler as shown in FIGS. 7 and 8. Although the loop mirror of FIG. 3 is bigger than the etched facet of FIG. 2, the loop mirror tends to be easier to fabricate. In the above two approaches, thermal electric (TE) cooling (for example, using a heat sink) and temperature control are not needed since the lasing wavelength is determined by the narrow-band external reflector, filter, mirror  11  of FIG. 1 or another type of tuning element  162  of FIG.  14  and FIG. 3 with very low temperature sensitivity. 
     In another alternative embodiment of the invention, as embodied herein and as shown in FIG. 4, the internal partial reflector is a distributed Bragg reflector (DBR)  406 . 
     EXAMPLE 
     The invention will be further clarified by the following example which is intended to be exemplary of the invention. 
     Example 1 
     The narrow-band internal partial reflector  40  of FIG. 1 is provided by the distributed Bragg reflector (DBR)  406  in the passive waveguide region  34  in FIG.  4 . The reflection band width (Δλ B ) can be written as          Δλ   B     ~       κ                   λ   2         π                 n                              
     where κ is the grating coupling coefficient, λ is the wavelength and n is the refractive index. The reflection bandwidth is about 2 nm assuming that λ=1.55 μm and κ=100/cm. The lasing wavelength is determined by the external filter  62  with a bandwidth much narrower than that of the internal DBR reflector  406 . In this approach, the chip fabrication process is exactly the same as for the modulator-integrated DFB laser except that the grating is located on the part of the waveguide  34  layer instead of located on the top of the active layer  22 . Since the internal reflection band is narrow, the temperature of the chip needs to be controlled to ±10° C. to ensure that the internal reflection band overlaps the center wavelength of the external filter  162 . However, wavelength monitoring to provide active feedback control is not needed. 
     In an additional embodiment, as embodied herein and as shown in FIGS. 5 and 6, the optical transmitter further includes a saturable absorber  72  grown on the same active layer as the gain element  16  and disposed between the gain element  16  and the partial reflector in the form of an etched facet  402  in FIG. 5 and a loop mirror in FIG. 6 or any other forms of a suitable partial reflector. The wavelengh-locked lasers of FIGS. 5 and 6 are now modified to form mode-locked lasers. The same fabrication process, as above, can also be applied to build such a mode-locked laser with an integrated modulator for time-division multiplexing (TDM) system applications. The extra reverse biased saturable absorber  72 , made of or grown on the same active layer as the gain element  16 , is integrated on the chip to initiate the mode-locking at a rate determined by the round trip time of the light in the external cavity  32 . The repetitive narrow pulse is then modulated by the integrated modulator  14  to transmit the signal. The outputs from several such mode-lock lasers can be interleaved in the time domain to increase the total information throughput. A significant cost saving thus comes from eliminating the optical pigtailing normally required of a discrete modulator. 
     It will be apparent to those of ordinary skill in the pertinent art that modifications and variations can be made to the external cavity  32  of the present invention depending on the type of external cavity lasers that is desired. For example, the external cavity of FIGS. 1-6, instead of being an air cavity can be implemented as a fiber cavity of FIG.  13 . 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit and scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.