Abstract:
A laser, including a grating structure consisting of two or more gratings generating a plurality of different wavelength peaks for reflection of optical radiation therefrom. The laser also includes a semiconductor device having a gain region which is operative to amplify the optical radiation, and a wavelength tunable filter (WTF) region which is adapted to filter the optical radiation. The device is optically coupled to the grating structure so as to define a laser cavity having a plurality of cavity modes. The cavity modes are selected by tuning a wavelength pass-band of the WTF region to overlap with one of the wavelength peaks of the grating structure.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 60/277,059, filed Mar. 20, 2001, which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to semiconductor lasers, and specifically to tuning of semiconductor laser systems. 
     BACKGROUND OF THE INVENTION 
     The increase in demand for greater bandwidths in communications is driving interest in semiconductor laser systems. In order to accommodate the high bandwidths, a typical system may use 80 or more wavelength division multiplexed (WDM) channels, each channel being optically time division multiplexed (OTDM) at rates of 40 Gbit/s or more. Such systems are required to generate narrow pulses, having widths of the order of several picoseconds. Semiconductor laser chips can generate coherent radiation at wavelengths of the order of 1.5 μm (approximately 200 THz), and so can form an integral part of such a system. However, a drawback common to all monolithic semiconductor lasers is that control of their operating wavelengths, repetition rates, and pulse widths, is not sufficiently accurate for the WDM/OTDM system described above. 
     FIG. 1 is a schematic diagram of a semiconductor laser system  10 , known in the art, which overcomes some of the drawbacks described above. A system similar to that of FIG. 1 is described in U.S. Pat. No. 5,305,336 to Adar et al. which is incorporated herein by reference. System  10  comprises a single-section semiconductor laser device  12  having a substantially 100% reflecting facet  14 , and an antireflection-coated facet  16 . Radiation from facet  16  is coupled into a fiber optic  18 , which has a Bragg grating  20  inscribed in the optic. In some embodiments known in the art, grating  20  comprises a multi spectral features fiber Bragg grating (MSFFBG). Grating  20  acts as a second partial reflector, causing device  12  and section  22  of the fiber optic to function as a fiber grating laser (FGL) that generates coherent radiation at a wavelength defined by the grating. 
     In the system described by Adar et al, 20 ps pulses at repetition rates of 2.5 GHz were produced by actively mode-locking the cavity, forming a mode-locked FGL (ML-FGL). The linear chirp of the grating allowed tuning of the repetition rate to a desired frequency. However, this was also accompanied by self-tuning of the emission wavelength of the laser over the width of the grating. Furthermore, the length of the pulses produced, and use of the single-section laser device which was modulated as a whole, limit the repetition rate. 
     SUMMARY OF THE INVENTION 
     It is an object of some aspects of the present invention to provide apparatus and a method for producing high repetition rate optical pulses. 
     In a preferred embodiment of the present invention, a laser is implemented by optically coupling a monolithic device having an active semiconductor lasing region with a multi spectral features fiber Bragg grating (MSFFBG) inscribed in a fiber optic. The laser is able to support a plurality of longitudinal modes of vibration. The device comprises a semiconductor wavelength tunable filter (WTF) which acts as a relatively wide band-pass filter, enabling the laser to be tuned to a number of adjacent modes to the virtual exclusion of the others. Preferably, the device also comprises a saturable absorber (SA) which is modulated with a radio-frequency signal and which is situated in an operating section of the device so that the laser is mode-locked to generate short pulses. Combining the active lasing region, the saturable absorber, and the WTF in the monolithic device, and optically coupling the device to the MSFFBG, forms an efficient compact lasing system that is tunable and that is able to generate short optical pulses at a specific wavelength with a high repetition rate. 
     In some preferred embodiments of the present invention, the monolithic device also comprises a phase-change region and a passive waveguide region. Addition of these two regions to the operating section of the device enables the SA region to be accurately positioned, in a two step process, at an optical center of a cavity defined by the device and the MSFFBG. In a first step the SA region is physically implemented at an approximate optical center. In a second step the phase-change region is tuned to adjust a phase delay within the cavity so that the SA region is accurately at the optical center. 
     The WTF may be implemented either as a transmission filter or as a reflection filter. If implemented as a transmission filter, the WTF is preferably formed as a grating assisted co-directional coupler, which may be tuned using current injection and/or by changing the temperature of the WTF. As a transmission filter, the WTF may be positioned substantially anywhere within the operating section of the monolithic device. 
     If the WTF is implemented as a reflection filter, it is most preferably positioned adjacent to an end facet of the device, acting there as a highly reflecting mirror. The reflection WTF is preferably implemented as a distributed Bragg reflector (DBR), which may be tuned using current injection and/or by changing the temperature of the DBR. Alternatively, the reflection WTF is implemented as a multi spectral features Bragg grating (MSFBG), which may be tuned by methods known in the art. 
     There is therefore provided, according to a preferred embodiment of the present invention, a laser, including: 
     a grating structure, including two or more gratings generating a first plurality of different wavelength peaks for reflection of optical radiation therefrom; and 
     a semiconductor device, including a gain region which is operative to amplify the optical radiation, and a wavelength tunable filter (WTF) region which is adapted to filter the optical radiation, the device being optically coupled to the grating structure so as to define a laser cavity having a second plurality of cavity modes, which are selected by tuning a wavelength pass-band of the WTF region to overlap with one of the wavelength peaks of the grating structure. 
     Preferably, the semiconductor device includes a saturable absorber which is adapted to be modulated so as to pulse the optical radiation. 
     Further preferably, the semiconductor device includes a highly reflective coated facet and an anti-reflection coated facet which bound the device, and the saturable absorber is positioned adjacent one of the facets. 
     Preferably, the semiconductor device includes an active phase-change region and a passive waveguide region which are adapted to position the saturable absorber centrally within an optical length of the laser cavity. 
     Further preferably, the active phase-change region implements a phase delay within the laser cavity so as to locate the saturable absorber at an optical center of the laser cavity. 
     Preferably, the WTF is implemented as a transmission band-pass filter. 
     Preferably, the semiconductor device includes an anti-reflection coated facet, and the WTF is implemented as a reflection band-pass filter located adjacent the anti-reflection coated facet. 
     Preferably, the grating structure includes a multi spectral features fiber Bragg grating (MSFFBG) inscribed in a fiber optic. 
     Further preferably, a width of a spectral feature of the MSFFBG is adjusted so as to determine a number of the plurality of the cavity modes. 
     There is further provided, according to a preferred embodiment of the present invention, a method for generating a laser output, including: 
     providing a grating structure generating a first plurality of different wavelength peaks for reflection of optical radiation therefrom; 
     optically coupling a semiconductor device to the structure so as to define a laser cavity, the device comprising a gain region which is operative to amplify the optical radiation and a wavelength tunable filter (WTF) region which is adapted to filter the optical radiation; and 
     tuning a wavelength pass-band of the WTF region to overlap with one of the wavelength peaks of the grating structure so as to generate a laser output in a second plurality of cavity modes defined by the overlap. 
     Preferably, the semiconductor device includes a saturable absorber (SA), and including modulating the SA so as to pulse the optical radiation. 
     Further preferably, the semiconductor device includes a highly reflective coated facet and an anti-reflection coated facet which bound the device, and including positioning the saturable absorber adjacent one of the facets. 
     Preferably, the method includes locating an active phase-change region and a passive waveguide region within the semiconductor device so as to position the saturable absorber centrally within an optical length of the laser cavity. 
     Further preferably, the method includes utilizing the active phase-change region to implement a phase delay within the laser cavity so as to locate the saturable absorber at an optical center of the laser cavity. 
     Preferably, the WTF is implemented as a transmission band-pass filter. 
     Preferably, the semiconductor device includes an anti-reflection coated facet, and the WTF is implemented as a reflection band-pass filter located adjacent the anti-reflection coated facet. 
     Preferably, the grating structure includes a multi spectral features fiber Bragg grating (MSFFBG) inscribed in a fiber optic. 
     Further preferably, the method includes adjusting a width of a spectral feature of the MSFFBG so as to determine a number of the second plurality of the cavity modes. 
     Preferably, optically coupling the semiconductor device to the grating structure includes butting the device to the structure. 
     Preferably, optically coupling the semiconductor device to the grating structure includes positioning a lens intermediate the device and the structure. 
     Further preferably, the grating structure includes a multi spectral features fiber Bragg grating (MSFFBG) inscribed in a fiber optic, and the lens is integral to an end of the fiber optic. 
     Preferably, tuning the resonant wavelength includes varying a temperature of the WTF region. 
     Alternatively, tuning the resonant wavelength includes varying a current injected into the WTF region. 
     Preferably, the grating structure is implemented to determine a number of the second plurality of the cavity modes, so as to control a pulse width of the optical radiation. 
     The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings, in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a semiconductor laser system known in the art; 
     FIG. 2 is a schematic sectional drawing of a tunable pulse generating laser system, according to a preferred embodiment of the present invention; 
     FIG. 3 shows schematic graphs of intensity vs. wavelength relationships for different elements of the system of FIG. 2, according to a preferred embodiment of the present invention; and 
     FIG. 4 is a schematic sectional drawing of an alternative tunable pulse generating laser system, according to a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Reference is now made to FIG. 2, which is a schematic sectional drawing of a tunable pulse generating laser system  30 , according to a preferred embodiment of the present invention. System  30  comprises a monolithic semiconductor device  31 , which is implemented with inert sections  32 , and an operational section  33  between sections  32 . Device  31  is bounded by two parallel facets; a first facet  42  is most preferably coated with a highly reflective coating, and a second facet  44  is coated with an anti-reflection coating. Section  33  comprises a gain region  50 , a wavelength tunable filter (WTF) region  48 , and a saturable absorber (SA) region  46 . Regions  46  and  50  are preferably formed by respectively structuring the two regions of section  33  according to the function of the region, by methods known in the semiconductor art. 
     WTF region  48  is most preferably implemented as a grating assisted co-directional coupler (GACC), and acts as a transmission band-pass filter. Device  31  is implemented so that SA region  46  abuts facet  42 . WTF region may be positioned anywhere between SA region  46  and facet  44 , and by way of example is assumed to be between gain region  50  and region  46 . Most preferably, a length of gain region  50  is substantially greater than a combined length of WTF region  48  and SA region  46 . Typically, a length of WTF region  48  and a length of SA region  46  are each of the order of 30 μm, and a total length of device  31  is of the order of 300 μm-1000 μm. 
     A ground electrode  34  is implemented on a lower face  35  of device  31 . An upper face  37  of device  31  has three separate electrodes  36 ,  38 ,  40  implemented thereon, correspondingly respectively with gain region  50 , WTF region  48 , and SA region  46 . Each region may be separately activated by its respective electrode. 
     System  30  also comprises a fiber optic  64  within which is implemented a multi spectral feature fiber Bragg grating (MSFFBG)  66 . MSFFBG  66  is most preferably formed from a plurality of discrete fiber gratings known as a super-structure grating (SSG)  56 , which is inscribed within the fiber optic by methods known in the art. Preferably, a lens  52  is formed as an integral part of a first end  68  of fiber optic  64 , end  68  being closest to facet  44 . During operation of system  30 , which is described in more detail below, lens  52  couples radiation between gain region  50  and MSFFBG  66 . Alternatively, other methods known in the art for coupling MSFFBG  66  with gain region  50  are used. For example, lens  52  may comprise one or more lenses distinct from fiber optic  64 , or end  68  of the fiber optic may be butted, with or without a mode converter, directly to facet  44 . MSFFBG  66  acts as a semi-reflecting mirror for system  30 , effectively forming a cavity  72  between the MSFFBG and facet  42 . Radiation generated within cavity  72  is transmitted from the MSFFBG and is output at a second end  70  of fiber optic  64 . 
     In operation, SA region  46  is activated by a DC reverse bias and a radio-frequency (RF) modulation being applied to electrode  40 ; WTF region  48  is activated by DC current injection at electrode  38 ; and gain region  50  is activated by applying DC excitation to electrode  36 . Mode-locked coherent pulses are produced by system  30  acting as cavity  72 , the cavity having longitudinal modes of vibration which are maintained between facet  42  and an effective length associated with MSFFBG  66 . 
     The combination of DC bias and RF modulation applied to SA region  46  produces a periodic absorption in the region, resulting in a short time interval during which system  30  experiences net gain. The short time interval is further shortened by SA region  46  being positioned adjacent to high reflection coated facet  44 , causing pulses generated within device  31  to collide with their reflections. The overall effect of the positioning of SA region  46 , and current flow in the region, is that the system is able to produce pulses having widths of the order of tens of picoseconds or less, when appropriate RF modulation is applied. 
     WTF region  48  acts as a transmission filter having a relatively wide band-pass. Tuning of a central frequency of the filter is most preferably implemented by varying current injected into electrode  38 . Alternatively or additionally, the filter is tuned by changing its temperature. Temperature variation may be implemented by any convenient method known in the art, such as by forming a small resistor  49  in place of and/or in addition to electrode  38 , or in a region of device  31  close to section  48 , and using the resistor as a heating element. 
     FIG. 3 shows schematic graphs of intensity vs. wavelength relationships for different elements of system  30 , according to a preferred embodiment of the present invention. A graph  100  corresponds to an overall gain curve of system  30 , the curve being a function of individual gains of components of the system. Cavity  72  has a multitude of longitudinal cavity modes like  102 A,  102 B,  102 C, . . . separated by Δλ, with wavelengths which are a function of an optical length of the cavity and the number of half-wavelengths comprising the mode. A graph  104  corresponds to the overall resonant curve of MSFFBG  66 , wherein each peak  104 A,  104 B,  104 C, . . . of the graph is a relatively narrow resonant curve of corresponding SSG  56  at respective central wavelengths λ A , λ B , λ C , . . . . 
     A graph  106  corresponds to the wavelength pass-band of WTF region  48 . WTF region  48  is implemented so that its wavelength pass-band substantially encloses only one of the peaks of graph  104 . Thus in FIG. 3, only longitudinal cavity modes within peak  104 B at λ B , such as modes  102 J,  102 K, and  102 L, will resonate since WTF region  48  is tuned to this wavelength region. Modes such as  102 N,  102 P, and  102 Q will be substantially suppressed since they are on the wings of graph  106  and will not lase. As described above, WTF region  48  is tunable, so that for modes within peak  104 A to resonate the region is tuned to lower wavelength λ A . Similarly, for modes within peaks  104 C,  104 D, and  104 E to resonate, region  48  is respectively tuned to higher wavelengths λ C , λ D , λ E . Thus system  30  can be effectively scanned from λ A  to λ E  by tuning WTF region  48  across the same wavelength range. The control of the number of adjacent longitudinal modes which lase defines the width of the pulse emitted by system  30 . The number of modes can be controlled by adjusting the width of the spectral features of the MSFFBG. 
     FIG. 4 is a schematic sectional drawing of an alternative tunable pulse generating laser system  130 , according to a preferred embodiment of the present invention. Apart from the differences described below, the operation of system  130  is generally similar to that of system  30  (FIG.  2 ), so that elements indicated by the same reference numerals in both systems  130  and  30  are generally identical in construction and in operation. A semiconductor device  131  has WTF region  48  positioned adjacent highly reflective coated facet  42 , SA region  46  positioned adjacent anti-reflection coated facet  44 , and gain region  50  positioned between the WTF region and the SA region. Regions  46 ,  48 , and  50  are activated by their respective electrodes  40 ,  38 , and  36 , substantially as described above for device  31 . 
     Device  131  further comprises an active phase-change region  134 , activated by an electrode  136  on upper face  37  of the device, in section  33 . A passive waveguide region  132  is also implemented in section  33 . 
     In contrast to device  31 , SA region  46  of device  131  is positioned approximately centrally within a cavity  172  formed between facet  42  and an effective length associated with MSSFBG  66 , by passive waveguide region  132  having its length implemented accordingly. During operation of device  131 , a phase delay introduced by phase-change region  134  is fine tuned, by adjusting current injected at electrode  136 , so that SA region  46  is effectively located at an optical center of cavity  172 . Positioning SA region  46  at the optical center of cavity  172  has substantially the same effect on pulses within the cavity as positioning the region adjacent to reflecting facet  42  of the cavity. That is, the pulses are shortened due to the fact that counter propagating pulses collide within SA region  46 . 
     It will be appreciated that since WTF region  48  acts as a transmission band-pass filter, it may be positioned substantially anywhere within section  33 , providing SA region  46  may be positioned at the optical center of cavity  172 . As for device  31 , transmission WTF region  48  in device  131  is tuned by current injection at electrode  38 , and/or temperature change of the region. 
     In an alternative embodiment of system  130 , WTF region  48  is implemented as a reflection type filter, most preferably by implementing the filter as a distributed Bragg reflector (DBR) or as a multi spectral features Bragg grating (MSFBG), for example, in the case of an MSFBG, as an SSG. When WTF region  48  acts as a reflector, it is positioned adjacent to facet  42 , and in this case facet  42  is anti-reflection coated. If reflection WTF region  48  is implemented as a DBR, it is preferably tuned by current injection via electrode  38 , and/or by temperature change of the region. If reflection WTF region  48  is implemented as an MSFBG, it is preferably tuned by methods which are known in the art. 
     It will be appreciated that the preferred embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.