Abstract:
A laser source for generating amplified and filtered optical output, comprising a VCSEL, a power optical amplifier, and a filter. A laser source for generating amplified and filtered optical output, comprising a first mirror and a second mirror forming a cavity, an optical amplifier disposed in the cavity, and filter means for filtering ASE generated and amplified by the optical amplifier. A system for generating amplified and filtered optical output, comprising an optical platform having electrical connections and a fiber optic connection, a VCSEL configured to generate seed light, an optical amplifier configured to receive and amplify seed light to generate power boosted ASE and a filter configured to reduce background noise from the power boosted ASE. A method of generating optical output having high optical power with high spectral fidelity, comprising generating seed light, amplifying seed light, and filtering the amplified optical output to reduce background noise.

Description:
REFERENCE TO PENDING PRIOR PATENT APPLICATIONS 
     This patent application claims benefit of: 
     (1) pending prior U.S. Provisional Patent Application Ser. No. 60/454,096, filed Mar. 12, 2003 by Kevin J. Knopp et al. for LASER SOURCE FOR RAMAN SPECTROSCOPY APPLICATIONS; and 
     (2) pending prior U.S. Provisional Patent Application Ser. No. 60/454,037, filed Mar. 12, 2003 by Kevin J. Knopp et al. for HIGH SPECTRAL FIDELITY LASER SOURCE WITH LOW FM-TO-AM CONVERSION AND NARROWBAND TUNABILITY 
     The two above-identified patent applications are hereby incorporated herein by reference 
    
    
     FIELD OF THE INVENTION 
     This invention is related to laser apparatus and method in general, and more particularly to apparatus and methods for generating optical output having high optical power and high spectral fidelity. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a laser source having optical output with high optical power and high spectral fidelity. 
     Another object of the invention is to provide a co-packaged low power seed signal generator and a power optical amplifier for performance, size, and cost advantages. 
     A still further object is to provide a method for generating optical output power with high spectral fidelity. 
     With the above and other objects in view, as will hereinafter appear, there is provided a laser source for generating amplified and filtered optical output having high optical power with high spectral fidelity, the laser source comprising: 
     a VCSEL configured to generate seed light having a given spectral wavelength; 
     a power optical amplifier configured to receive the seed light generated by the VCSEL and amplify the seed light so as to generate amplified optical output having a given output power; and 
     a filter configured to receive the amplified optical output from the power amplifier and reduce background ASE from the power optical amplifier so as to generate the amplified and filtered optical output having high optical power with high spectral fidelity. 
     In accordance with a further feature of the invention there is provided a laser source for generating amplified and filtered optical output having high optical power and having high spectral fidelity, the laser source comprising: 
     a first mirror and a second mirror forming a cavity therebetween; 
     an optical amplifier disposed in the cavity formed between the first mirror and the second mirror, the optical amplifier configured to generate ASE and amplify the power of the generated ASE between the first mirror and the second mirror; and 
     filter means for filtering the ASE generated and amplified by the optical amplifier to reduce background noise therefrom so as to generate the amplified and filtered optical output laser having high optical power and high spectral fidelity. 
     In accordance with a further feature of the invention there is provided a system for generating amplified and filtered optical output having high optical power and high spectral fidelity, the system comprising: 
     an optical platform having a set of electrical connections and a fiber optic connection; 
     a VCSEL configured to generate seed light, and the VCSEL in electrical connection to one of the set of electrical connections of the optical platform; 
     an optical amplifier configured to receive the seed light generated by the VCSEL and amplify the seed light so as to generate power boosted ASE having a given output power, and the optical amplifier in electrical connection to one of the set of electrical connections of the optical platform; and 
     a filter configured to receive the power boosted ASE from the power amplifier and reduce background noise from the power boosted ASE so as to generate an output ASE having high spectral fidelity. 
     In accordance with a still further feature of the invention there is provided a method of generating optical output having high optical power with high spectral fidelity, the method comprising: 
     generating seed light from a low power source, the seed light having a given output power and a given spectral fidelity; 
     amplifying the seed light source from the given output power to an amplified optical output using a power optical amplifier, the amplified optical output having an adjusted spectral fidelity and an amplified output power, and the amplified output power being greater than the given output power of the seed light; and 
     filtering the amplified optical output produced by the optical amplifier to reduce background noise therein so as to generate the amplified and filtered optical output having high spectral fidelity greater than the adjusted spectral fidelity of the power boosted ASE. 
     The above and other features of the invention, including various novel details of construction and combinations of parts and method steps will not be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular devices and method steps embodying the invention are shown by way of illustration only and not as limitations of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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 are to be considered together with the accompanying drawings wherein like numbers refer to like parts, and further wherein: 
         FIG. 1  is a schematic diagram of a laser source of a preferred embodiment of the present invention; 
         FIG. 2  is a schematic diagram of a perspective view of the laser source shown in  FIG. 1 ; 
         FIGS. 3A ,  3 B and  3 C are a diagrammatic view, a side diagrammatic view and an end diagrammatic view of the laser source shown in  FIG. 1 , respectively; 
         FIG. 4  is a schematic diagram of an optical component comprising the laser source of  FIG. 1  together with an electrical connector (not shown) and an optical fiber connector; 
         FIGS. 5A–5D  are a top diagrammatic view, a side diagrammatic view, and an end diagrammatic view of the optical component shown in  FIG. 4 ; 
         FIG. 6  is a diagrammatic illustration of the spectral properties of the laser source shown in  FIG. 1 ; 
         FIG. 7  is a schematic diagram of another preferred embodiment of the present invention with a laser source coupled to a SMF fiber; and 
         FIG. 8  is a schematic diagram of a perspective view of the laser source shown in  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     At the foundation of the present invention is a novel co-packaged seeded power optical amplifier (CP-SPOA) technology. Referring to  FIG. 1 , and in a preferred embodiment of the present invention, this novel technology comprises a co-packaged source module  5  which couples a low-power source  10  providing a seed optical signal  15  having the desired spectral characteristics into a long-cavity semiconductor waveguide  20  for power amplification. This co-packaged approach has tremendous advantages in performance, size, and cost. Some of these advantages of this technical platform include (1) design optimizations in that spectral and power performance are orthogonal, (2) higher yield from co-packaging rather than monolithic integration, (3) independent controls for spectral adjustments and power adjustments, and (4) compatibility with reliable telcom qualified packaging techniques. 
     The novel technology of the present invention is ideal for spectroscopy applications where a laser source&#39;s spectral fidelity, wavelength accuracy, AM-to-FM conversion ratio, output power, and reliability are primary concerns. 
     In addition, the present invention allows scalability to higher output powers without compromise of spectral performance. 
     Overview of Technical Approach 
     A schematic representation of a preferred embodiment of the present invention includes CP-SPOA source module  5  shown in  FIG. 1 . Seed light  15  is generated from a low-power VCSEL  10  which is then coupled into a power optical amplifier  20 . A TEC  35  is thermally connected with the VCSEL  10  to set the absolute wavelength of source module  5 . A second TEC  40  is used to maintain the temperature of the optical platform. A thin-film tap  45  and photodetector  50  provide power monitoring functionality so as to maintain output power stability of the source module  5 . An isolator  55  may be used to provide high optical return loss. The entire optical train is preferably contained in a 14-pin hermitically-sealed butterfly package  60  with either a multi-mode fiber pigtail  65  or a single-mode fiber pigtail  70  ( FIGS. 8 and 9 ). 
       FIG. 2  illustrates a preferred optical layout design within hermitically sealed butterfly package  60 . VCSEL light  15  provides the high spectral fidelity single longitudinal mode required for the seed signal. In a preferred embodiment of the present invention, seed light  15  has a side mode suppression ratio (SMSR) of greater than 20 dB and a linewidth of less than 100 MHz. In another preferred embodiment of the present invention, seed light  15  has a SMSR of greater than 30 dB and a line width of less than 10 MHz. Power optical amplifier  20  serves to boost seed light  15  to a desired output power. For example, the power of seed signal  15  may be boosted from 10 mW to 1 W. Superb wavelength stability is fundamentally achieved through the reliance on the stability of the optical index of the semiconductor cavity in a similar manner as a conventional Telecom grade DFB laser. The current to power optical amplifier  20  can be adjusted so as to control output power independent to spectral wavelength. 
     A filter  25  disposed within source module  5  reduces background noise from optical signal  20  so as to produce an optical output  30  having high power output and high spectral fidelity. 
     The output wavelength can be dynamically tuned through modulation of the seed current or through adjustment of the setpoint of seed TEC  35 . The FM-AM conversion experienced during tuning will be minimized through the use of a VCSEL as the seed and through saturation of the power optical amplifier. The estimated AM/FM ratio for the proposed device is ˜0.5%/GHz as opposed to ˜5%/GHz for a typical DFB solution. 
     The independence of the output power of the optical amplifier with respect to the spectral wavelength of the seed light enables the use of various “lock-in” techniques or modulation techniques and can also eliminate mechanical shutter. In a preferred embodiment of the present invention, filter  25  is a multicavity thin-film filter configured at the output of the laser source so as to reduce the background ASE from the laser emission by the greater than 70 dB, which in turn allows-potential detection of weaker Raman signals. Additionally, the single longitudinal mode nature of the seed source signal allows the elimination of Raman “ghost” signals. 
     In a preferred embodiment of the present invention, an optical platform and thermoelectric cooler (TEC) combination  40  supports and thermally regulates power optical amplifier  20  and filter  25 . 
     Referring to  FIGS. 1–3 , and in a preferred embodiment of the present invention, there is shown a schematic representation of a source module  5  having a co-packaged seeded power-optical amplifier (CP-SPOA)  5  ( FIG. 1 ), a 3-D rendering of a hermetically-sealed laser source module with the lid removed ( FIG. 2 ), and a dimensional layout of the hermetically-sealed laser source module  5  with illustrative dimensions in mm ( FIG. 3 ). 
     Laser Source Subsystem 
     Referring now to  FIG. 4 , and in a preferred embodiment of the present invention, source module  5  is integrated with driver electronics  75 A– 75 E to create a laser source component  80 . A rendering of the complete laser source component  80  is shown in  FIG. 4 . In  FIG. 5 , there is shown a dimensional layout of laser source component  80 . Laser source component  80  has an SMA fiber connector output  75 A and four electrical connections: a 5V supply voltage  75 B, a laser set-point input voltage  75 C, an output voltage proportional to the output optical power  75 D, and a ground pin  75 E. Component  80  operates to provide the output power in a constant power mode using an analog feedback loop for exceptional power stability over life. In  FIG. 4 , there is a schematic rendering of the laser source component  80  with a cut-away shown. In  FIGS. 5A–5D , there is a dimensional layout of the laser source component  80  with the dimensions shown in inches. 
     Optical Performance Specifications 
     In a preferred embodiment of the present invention, laser source module  5  conforms to the performance criteria outlined in Table 1 over its life in the environmental conditions specified in Table 4. The specifications for the final product, alpha prototypes, and beta units are listed. 
     Table 1 specifies preferred optical performance specifications achieved prior to end of life (EOL) of the laser source module  5 ; however, it should be appreciated that this table is provided by way of example only and not by way of limitation. 
     
       
         
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                   
                 Typ- 
                   
                   
                   
                 Final 
               
               
                 Parameter 
                 Unit 
                 Min 
                 ical 
                 Max 
                 α 
                 β 
                 Product 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Output Optical 
                 mW 
                 300 
                 350 
                   
                 ✓ 
                 ✓ 
                 ✓ 
               
               
                 Power 
               
               
                 Output Power 
                 % 
                   
                 0.5 
                 2 
                 ✓ 
                 ✓ 
                 ✓ 
               
               
                 Stability 1   
               
               
                 Wavelength 
                 nm 
                 782.0 
                 785.0 
                 788.0 
                 ✓ 
                 ✓ 
                 ✓ 
               
               
                 Peak Wavelength 
                 nm 
                   
                 &lt;0.01 
                 0.1 
                   
                 ✓ 
                 ✓ 
               
               
                 Stability 2   
               
               
                 Number of 
                 # 
                   
                   
                 Single 
                 ✓ 
                 ✓ 
                 ✓ 
               
               
                 Longitudinal 
                   
                   
                   
                 Mode 
               
               
                 Modes 
               
               
                 Laser Line Width 
                 MHz 
                   
                 3 
                 10 
                 ✓ 
                 ✓ 
                 ✓ 
               
               
                 Side Mode 
                 dB 
                 25 
                 30 
                   
                 ✓ 
                 ✓ 
                 ✓ 
               
               
                 Suppression Ratio 
               
               
                 Optical Signal-to- 
                 dB 
                 40 
                 45 
                   
                 ✓ 
                 ✓ 
                 ✓ 
               
               
                 Noise Ratio 3   
               
               
                 Width of ASE 
                 nm 
                   
                   
                 4 
                   
                 ✓ 
                 ✓ 
               
               
                 Suppression Filter 
                 (FW@ 
               
               
                   
                 70 dB) 
               
               
                 ASE Suppression 
                 dB 
                 70 
                 80 
                   
                   
                 ✓ 
                 ✓ 
               
               
                 Relative Intensity 
                 dB/Hz 
                   
                   
                 −100 
                   
                 ✓ 
                 ✓ 
               
               
                 Noise 
                   
                   
                   
                 f &lt; 1 
               
               
                   
                   
                   
                   
                 GHz 
               
               
                   
               
               
                   1 High stability is provided via a closed loop analog feedback loop with a time constant of &gt;100 kHz. 
               
               
                   2 Maximum change in wavelength from start-of-life through end-of-life across temperatures. Over a typical 8 hour time period the wavelength will have maximum drifts of &lt;&lt;0.01 nm. 
               
               
                   3 Measured 1 nm away from the peak with a resolution bandwidth of 0.1 nm 
               
             
          
         
       
     
     A depiction of the definitions of the spectral properties of module  5  is shown in  FIG. 6 . As shown, a thin-film multi-cavity filter is used to suppress the ASE background emission of the laser source by &gt;70 dB. 
     Mechanical Assembly 
     In a preferred embodiment of the present invention, the laser source module has the mechanical attributes as specified in Table 2 for the final product, alpha prototypes, and beta units. 
     Table 2 specifies preferred mechanical attributes of the laser source module  5 ; however, it should be appreciated that this table is provided by way of example only and not by way of limitation. 
                                                                     Final       Parameter   Unit   Value   α   β   Product                   Fiber Connector   Type   SMA for   ✓   ✓   ✓               50 μm MMF       Electrical Connector   Type   4-pin   ✓   ✓   ✓       Case Material   Type   Anodized   ✓   ✓   ✓               Aluminum       Dimensions of the   inch   2.5 × 3.5 × 1.125   ✓   ✓   ✓       Subsystem                    
Electrical Specifications
 
     In a preferred embodiment of the present invention, laser source module  5  has electrical requirements as specified in Table 3 for the final product, alpha prototypes, and beta units. 
     Table 3 specifies preferred electrical requirements of the laser source module  5 ; however, it should be appreciated that this table is provided by way of example only and not by way of limitation. 
                                                                                                       Typ-               Final       Parameter   Unit   Min   ical   Max   α   β   Product                                Subsystem Supply   V   4.8   5   5.2   ✓   ✓   ✓       Input   A       0.8   1.2   ✓   ✓   ✓       Laser Set-Point   mV/mW       10           ✓   ✓       Control Voltage       Power Monitor   mV/mW       10           ✓   ✓       Output Voltage       Output Power Slew   Hz   10               ✓   ✓       Rate 4         Output Power   kHz   100               ✓   ✓       Feedback Response 5         Power   W       4   6       ✓   ✓       Consumption 6                   4 The output optical power will be updated in response to a change in set-point voltage at a rate of 10 Hz.         5 The output optical power will be controlled in a constant power loop updated at a rate &gt;100 kHz.         6 Maximum power consumption when operating the subsystem at a case temperature of 40° C./0° C.            
Environmental Conditions
 
     The environmental operating conditions for the laser source component  80  are shown in Table 4. The heat dissipated from laser source  5  and TEC  40  within the optical package must be dissipated through mating of optical module  5  to an appropriate heat sink. 
     Table 4 specifies preferred environmental operating conditions for the laser source module  5 ; however, it should be appreciated that this table is provided by way of example only and not by way of limitation. 
     
       
         
               
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                   
                   
                   
                 Final 
               
               
                 Parameter 
                 Unit 
                 Value 
                 α 
                 β 
                 Product 
               
               
                   
               
             
             
               
                 Operating Temperature 
                 ° C. 
                    0 to 40 
                   
                 ✓ 
                 ✓ 
               
               
                 Storage Temperature Range 
                 ° C. 
                 −40 to 80 
                   
                 ✓ 
                 ✓ 
               
               
                   
               
             
          
         
       
     
     Laser Source Module 
     Laser source module  5  as shown in  FIG. 2  is the heart of component  80 . Module  5  is contained within the mechanical assembly of component  80 . Specifications on the performance of laser source module  5  are presented herein below. Most of these parameters are internal to the subsystem and are invisible to the end user. 
     Optical Performance Specifications 
     Laser source module  5  has the performance criteria outlined in Table 1 over its life in the environmental conditions specified in Table 7. The optical specifications of module  5  are identical to that for component  80  with the exception that an increased output power (+0.2 dB) is required to budget for connector loss and aging of the SMA. 
     Mechanical Assembly 
     The mechanical attributes of the laser source module are specified in Table 5 for the final product, alpha prototypes, and beta units. 
     Table 5 specifies preferred mechanical attributes of laser source module  5 ; however, it should be appreciated that this table is provided by way of example only and not by way of limitation. 
     
       
         
               
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                   
                   
                   
                 Final 
               
               
                 Parameter 
                 Unit 
                 Value 
                 α 
                 β 
                 Product 
               
               
                   
               
             
             
               
                 Fiber Type 
                 Type 
                 50 μm MMF 
                 ✓ 
                 ✓ 
                 ✓ 
               
               
                 Fiber Connector 
                 Type 
                 SMA 
                 ✓ 
                 ✓ 
                 ✓ 
               
               
                 Fiber Pigtail Length 
                 m 
                 1 
                   
                 ✓ 
                 ✓ 
               
               
                 Package Style of Optical 
                 Type 
                 14-Pin Butterfly 
                 ✓ 
                 ✓ 
                 ✓ 
               
               
                 Module 
               
               
                 Dimensions of Optical 
                 mm 
                 42 × 12 × 13 
                   
                 ✓ 
                 ✓ 
               
               
                 Module 
               
               
                 Sealing of Optical 
                 Type 
                 Hermetic 
                   
                 ✓ 
                 ✓ 
               
               
                 Module 
               
               
                   
               
             
          
         
       
     
     Electrical Specifications 
     The electrical requirements of the laser source module  5  are specified in Table 6 for the final product, alpha prototypes, and beta units. 
     Table 6 provides preferred electrical requirements of the laser source module  5 ; however, it should be appreciated that this table is provided by way of example only and not by way of limitation. 
                                                                                                       Typ-               Final       Parameter   Unit   Min   ical   Max   α   β   Product                                Seed Laser Driver   V   0   3   4       ✓   ✓           mA   0   5   20       ✓   ✓       POA Current Driver   V   0   2   2.3       ✓   ✓           A   0   1.0   1.5       ✓   ✓       POA TEC Driver   V   −1.5   0.4   1.5       ✓   ✓           A   −1.5   0.7   1.5       ✓   ✓       Power Dissipation 7     W       3.5   5       ✓   ✓       POA Thermistor   kΩ   9.5   10   10.5   ✓   ✓   ✓       Resistance (@ 25° C.)       Monitor Photodiode   nA           100       ✓   ✓       Dark Current       (V reverse  = 5 V)       Signal Power Monitor   μA/mW   3.8   4   4.2       ✓   ✓       Responsivity       (V reverse  = 5 V)                 7 Total Power Consumption with TEC at the highest/lowest operating case temperature.            
Environmental Conditions
 
     The environmental operating conditions are shown in Table 7. The heat dissipated from laser source  5  and TEC  35  within the optical module must be dissipated through mating of laser component  80  to an appropriate heat sink. There is a 5° temperature differential between the case of component  80  and the case of module  5 . 
     Table 7 specifies preferred environmental conditions for the laser source module  5 ; however, it should be appreciated that this table is provided by way of example only and not by way of limitation. 
     
       
         
               
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                   
                   
                   
                 Final 
               
               
                 Parameter 
                 Unit 
                 Value 
                 α 
                 β 
                 Product 
               
               
                   
               
             
             
               
                 Operating Temperature 
                 ° C. 
                 5 to 45 
                   
                 ✓ 
                 ✓ 
               
               
                 Storage Temperature Range 
                 ° C. 
                 −40 to 80  
                   
                 ✓ 
                 ✓ 
               
               
                 Operating Humidity Range 
                 % 
                 0 to 90 
                   
                 ✓ 
                 ✓ 
               
               
                   
               
             
          
         
       
     
     Qualification 
     The proposed laser source subsystem will be shown to have a mean time to failure (MTTF) of greater than 10,000 hours. End of life (EOL) has occurred when the specifications of Table 1 can no longer be met. Processes and techniques compatible with Telcordia qualification standards may be used to ensure reliable operation. Qualification testing preferably includes checks related to aging, storage, damp-heat, thermal cycling, and mechanical shock/vibration. Other tests will be performed as needed to ensure product quality.