Tunable Semiconductor Lasers by dfgh4bnmu

VIEWS: 15 PAGES: 61

									Abstract: Tunable semiconductor lasers continue to be in just about everyone’s list of important components
 for future fiber optic networks. Various designs will be overviewed with particular emphasis on the widely
                                           tunable (>32nm) types.




              Tunable Semiconductor Lasers

                                             a tutorial



                                      Larry A. Coldren
                University of California, Santa Barbara, CA
                                       coldren@ece.ucsb.edu
               Agility Communications, Santa Barbara, CA
                                         lcoldren@agility.com
                                 2




Contents
■   Why Tunable Lasers?

■   Basic Tuning Mechanisms

■   Examples of Tunable Lasers

■   Control of the Wavelength

■   Reliability Issues
                                              3




   Optical Network Architecture




        Core



                                  Edge
More bandwidth and services/$
Low-cost components and agile architectures
                                                                      4




    Introduction
■   Tunable lasers have been of great interest for some time
    − Dynamic networks with wavelength reconfigurability
       − Networking flexibility
       − Reduced cost
    − One time provisioning (OTP) and sparing seen as side benefits
■   Current market conditions….
    − More cautious approach from carriers and system vendors
    − OTP and sparing are now the leading applications
■   Tunable lasers are compared with DFB or EML
    − Important to do “apples to apples” comparison
       − Functionality
       − Performance
       − Total Cost of Ownership
                                                            5




Why Tunable Lasers?
■   One time provisioning—inventory and sparing

■   Field re-provisioning—new services without hardware
    change or truck roll

■   Reconfigurable Optical Add/Drop Multiplexers (ROADM)—
    Drop and add any channel without demux/mux

■   Wavelength conversion—Eliminates wavelength blocking
    without OEO line cards

■   Photonic Switching—Eliminates many OEO line cards

■   Wavelength Routing—Use passive optical core
                                              6




Applications –
One time provisioning—the universal source


■   Laser is provisioned once only

■   Simplifies manufacturing

■   Drastically reduces inventory

■   Minimizes sparing to a manageable level

■   Simplifies forecasting
                                                                            7




    Applications – Re-provisioning

■   Laser is provisioned many times remotely to set up
    new services
     − Seconds timeframe
     − Point and click or ultimately controlled automatically by software

■   Can only be addressed using a widely tunable laser
     − Without severe constraints

■   Drastically reduces inventory

■   Simplifies forecasting
                                                          8




Applications – Re-configurable OADM
                        Tunable filter elements


           in                    thru               out




                     drop                     add

                                         Tunable
                            Rx             Tx



■ Drop and Add without Demux and Mux of all channels
■ Must be “hitless” filter tuning
■ Eliminates mux/demux and OEO
■ Tunable lasers are a key enabler
                                                                               9




Applications – Photonic Switching 1

                  1   2       3
                                  LR Tx


                                          OR
    LR   SR                       SR Rx                1   2       3
    Rx   Tx                                    Rx Tx                       4

                                      4
                                                                           5
                                      5
                                                                           6
    Grey                              6
    Optics                                     Tunable         8       7
              8           7




■ Photonic switches require O-E-O on I/O to prevent blocking
■ Tunability reduces O-E-O requirements in half
■ Requires moderately fast switching (ms)
                                                                                   10




 Applications – Wavelength Conversion
 ■ Intersection of metro rings
 ■ Wavelengths transition between rings
       − in optical domain
 ■     Tunable lasers used to resolve wavelength blocking
       − Alternative is a bank of fixed wavelength lasers


Node                                                                        Node
 3B             Node                                                         3R
                                                            Node
                 4B
                                                             4R



       Node                                                          Node
        2B
                  Node                                        Node    2R
                   1B                                          1R

                                   Rx Tx
                                                                    11




Applications – Wavelength Routing
(Optical Packet Switching)

         Line cards with                     Line cards
         Tunable lasers
                   Tx


                                NxN
                              Lambda
                               Router
                               (AWG)




   ■ High capacity, high density router function—need wide tuning
   ■ Wavelength used to route traffic through passive device
   ■ For Packets requires very fast switching
                                 12




Contents
■   Why Tunable Lasers?

■   Basic Tuning Mechanisms

■   Examples of Tunable Lasers

■   Control of the Wavelength

■   Reliability Issues
                                                                       13




    Generic Single-Frequency Laser
                               Mode-selection
                  Gain            filter
                                                      Output


                                                           mλ/2 = nL
Mirror-1                                   Mirror-2


                                 Gain

                            Lasing mode
           Mode-selection
              filter
                                                      Possible modes

                                                      λ
                                                                                                             14




     Examples of Single-Frequency Lasers
                                                              gain
■   DFB                                                                              Grating (mode-selection
     −   All-elements combined and         Light Out                                 & distributed mirror)
         distributed along length
                                                                                     Gain region
                                                                DFB
                                                AR                                   HR
                                                       Gain                Rear Mirror
■   DBR                                    Light Out
     −   Elements separated with
         individual biases
                                                                            DBR

                                                                      AR                 Ext. grating mirror
                                                       Gain
■   External Cavity                        Light Out
     −   Gain block + external lens &
         grating


                                                                           Collimating lens

■   VCSEL
                                                                                  Active
     −   Short cavity for mode selection
                                                                                  DBR mirrors


                                                                     Light Out
                                                                          15




  How Tunable Lasers Tune
Mode wavelength:
                                                Effective Cavity length
                     mλ/2 = nL
    Mode number
                                      Effective index
                    Wavelength


Relative change in wavelength:
                                              Tuned by mode-selection filter
                   ∆λ = ∆n + ∆L - ∆m          (via index or grating angle)
                    λ    n    L m
Tuned by net cavity index change
                                   Tuned by physical length change
                                                                             16




Generic Tunable Single-Frequency Laser
                                                                   Tunable
                   Gain                    Mode-
                                                                    output
                                Cavity    selection
                                phase       filter
                                 (∆n)       (∆m)            (∆L)

Mirror-1                                              Mirror-2

                                                            mλ/2 = nL
                                 Gain

           Mode-selection   Lasing mode
            filter (∆m)
                                                      Possible modes(∆n, ∆L)

                                                       λ
                                                                                                        17




    Solutions for Tunable Lasers
■   DBR Lasers                                            Gain Phase Rear Mirror
                                          Light Out
     − Conventional DBR (<8 nm)
     − Extended Tuning DBR’s (≥ 32 nm)                                        DBR


■   External Cavity Lasers (≥ 32 nm)
     − Littman-Metcalf/MEMs
     − Thermally tuned etalon                                                        Light Out


                                                               Light Out
■   MEMS Tunable VCSEL (< 32 nm)
     − Optically or electrically pumped


                                                                                      W in d o w

■   DFB Array (3-4 nm X #DFBs)
                                                       NEC
     − On-chip combiner + SOA
     − Or, off-chip MEMs combiner                                                8 M ic r o a r r a y
                                                                                 D F B -L D s
                                                                  MMI
     − Thermally tuned                                                         S -b e n t
                                                                               w a v e g u id e s
                                                  W in d o w   SOA

                                                                           C h ip s iz e :
                                                                           0 .4 x 2 .1 5 m m    2
                                 18




Contents
■   Why Tunable Lasers?

■   Basic Tuning Mechanisms

■   Examples of Tunable Lasers

■   Control of the Wavelength

■   Reliability Issues
                                                                               19




    Examples of Tunable Lasers
■   Narrowly tunable (not discussed further)
    − Temperature tuned DFBs ~ 3nm
    − Narrowly tunable 2 or 3 section DBR lasers   ~ 8nm
■   DFB selectable arrays
    − Select DFB array element for coarse tuning + temperature tune for fine
      cavity mode tuning
    − Integrated on-chip combiners + SOAs or off-chip MEMs deflectors
■   External-cavity lasers
    − External grating reflector for mode-selection filter
    − Angle-tune mirror for mode selection—coarse tuning
    − Change length and/or phase section for fine tuning
■   MEMS Tunable VCSELs
    − Move suspended top mirror by electrostatic or thermal tuning
    − Single knob tuning for both coarse and fine
■   Widely tunable DBR lasers
    − Coarse tuning by index tuning of compound mirrors/couplers
    − Fine tuning by index tuning of phase section
    − Dual SGDBR or vertical-coupler + SGDBR mode selection filters
                                                                                    20



Wavelength-selectable light sources (WSLs)
for wide-band DWDM applications
                                                                        Window

 Feature
   DFB-LD-array-based structure
                                                                     8 Microarray
   Wide-band tunability                                    MMI       DFB-LDs
                                                                    S-bent
   Compact & stable                                Window SOA
                                                                    waveguides

   Multi-λ locker module                                         Chip size:
                                                                 0.4 x 2.15 mm2

 Performance                                     Schematic of wide-band WSL


   WSLs for S-, C-, L- bands (OFC’02)
    8 array, ∆λ ~ 16 nm (∆T = 25K) x 6 devices
   Multi λ-locker integrated
   Wide-band WSL module (OFC’02)
   ∆λ ~ 40 nm (∆T = 45K)                           Multi λ−locker integrated
                                                   Wide-band WSL module
                                                                                                                                21



                  WSLs for S-, C-, L- bands applications
                  - Lasing spectra -
                  20
                  10               S -b a n d                        C -b a n d                        L -b a n d

                    0
Intensity (dBm)




                  -1 0
                  -2 0
                  -3 0
                  -4 0
                  -5 0
                  -6 0            S1        S2                    C1          C2                      L1            L2
                                                 W a fe r 1                              W a fe r 2
                  -7 0
                         1470       1490         1510         1530            1550            1570          1590         1610
                                                               W a ve le n g th (n m )


                                ∆λ ~ 16 nm (∆T 25K) @15 - 40 ℃
                                6 devices → 135 channels @100-GHz ITU-T grid
                                SMSR > 42 dB
                                Pf > ~ 10 mW @ IDFB= 100 mA, ISOA= 200 mA
                                             22




Fujitsu DFB Array Integrated Tunable Laser




                            0.5 × 1.8 mm




Fujitsu Laboratories Ltd.
                                                             23




  Fujitsu Wavelength Tuning Characteristics




            Temperature tuning   Spectra at 32 wavelengths


Fujitsu Laboratories Ltd.
                                                                                 24




    Santur Switched DFB Array
                1 mm




  12 element DFB array, each temperature
tuned 3nm for 36nm total tuning range -
only one laser on at a time
  MEMS mirror couples the selected laser
to fiber

 Advantages:
      DFB characteristics (optical quality, reliability, wavelength stability)
      No SOA, tuning sections, phase-sensitive mechanics
      High yield, low cost passive alignment (MEMS does the rest)
      Built-in shutter/VOA
                                                                            25




Santur 20 mW Module Performance




Full band tunability (36nm C-band, 42nm L-band)
Built-in wavelength locker (25GHz channel spacing)
>50dB SMSR, 2MHz linewidth
Typical tuning time ~ 2sec
Resistant to shock and vibe with no servo (10G causes < 0.2dB fluctuation
in power)
                                                                        26




    Intel External-Cavity Approach
    (acquired from New Focus)

•   Double sided external cavity laser design, well known in test and
    measurement applications
•   Temperature tuned etalon replaces mechanical tuning device
•   No moving parts, but challenging packaging requirements
                                                                                27




Littman-Metcalf Cavity (after New Focus)
    HR               Laser Diode
   Coating              Chip
                                   Collimating Lens


        AR                                            Retroreflector
      Coating


     Pivot
     Point

             Wavelength                                                Laser
              Tuning                                                   Output



                                              Diffraction
                                               Grating
                              28



Iolon External-Cavity Laser
with MEMs Mirror Movement
                                        29




    Tunable VCSELs (optically pumped)
■   Cortek-Nortel-Bookham?

■   Component technologies
     −   MEMS
     −   Thin Film
     −   InP Laser
     −   Packaging

■   Advantages:
     − High Power
     − Wide Tuning Range
     − Continuously Tunable
30
                                                                                              31




Agere “Narrowly” Tunable DBR/SOA/EAM

  EA-DBR Operation                                                Tuning Current
                                                                  High                 Low


                                                        L
     Gain         Tuning     EA Modulator
   Bragg mirror select FP mode
   Tuning current moves Bragg mirror
                                                                              λ
         A Five Stage Bell Labs Design




              Gain           DBR             Optical           Detector           Modulator
             Section         Mirror         Amplifier       “Power Monitor”



                             Many more 7 – 10 nm designs
                           32




Extended tuning range:
SSGDBR--NEL
Phase modulated gratings
                                                                                                                                           33




Extended tuning range:
GCSR--ADC-Altitun
  SGDBR + GACC


   Gain          Coupler    Phase   Reflector S-DBR                                        30


 400µm           600µm      150µm      900µm                                               25




                                                      R fle to c rre t [m ]
                                                       e c r u n A
                                                                                           20


p-InP                                                                                      15

                                                                                           10

                                                                                           5
n-InP
                                                                                           0

QWs structures       λg = 1.3 µm      λg = 1.38 µm
                                                                                            1515    1525   1535       1545   1555   1565
                                                                                            8

λg = 1.55 µm                                                                                7




                                                                     C u le c rre t [m ]
                                                                      op r u n A
                                                                                            6
                                                                                            5
                                                                                            4
                                                                                            3
                                                                                            2
                                                                                            1
                                                                                            0
                                                                                             1515   1525   1535       1545   1555   1565
                                                                                                           Wavelength [nm]
Agility’s Extended Tuning Range Technology:
      Widely Tunable SGDBR Lasers
                                                                                                                               35




    Sampled Grating Tunable Lasers
                                                                               10
                     Front                   Rear
         Amplifier   Mirror   Gain   Phase   Mirror                             0                      24 1             3




                                                      Laser Emission, dBm
                                                      Relative Power (dB)
                                                                              -1 0

                                                                              -2 0

  Light Out                                                                   -3 0

                                                                              -4 0

                                                                                 1
                                                                              -5 0
                                                                                            B ack
                                                                                            F ro n t
■ 5-10X Tuning Range of DBR                                                   0 .8




                                                        Mirror Reflectivity
■ Reliable, Manufacturable InP Technology
                                                                              0 .6
■ Can Cover C band, L band or C + L
                                                                              0 .4


                                                                              0 .2


                                                                                0
                                                                                     1530      1540 1550 1560           1570
                                                                                               W a v elen g th (n m )
                                                                     36




Advantages of Monolithic Integration
Widely Tunable SG-DBR Laser with integrated SOA and EAM
                                    Front                   Rear
            Modulator   Amplifier   Mirror   Gain   Phase   Mirror




Light Out




  EA Modulator             SOA               SG-DBR Laser


   Advantages:
    smaller space (fewer packages)
    lower cost (fewer package components)
    lower power consumption (lower coupling losses)
    high reliability (fewer parts)
                                                                                                                                                          37



                  Fast Wavelength Switching
                  of SGDBR Lasers
                                                                 Packet Switching Applications
                                                                                        100
Voltage




                  Electronic                                                                                    Optical signal at final ITU +/- ~10 GHz
                  Trigger                       Switching time = 10 ns
                                                                                            80
Light Power




                               Channel 50 on                                                60




                                                                                    Count
                                                                  Channel 50 off
                                                                                            40
Light Power




                                                                  Channel 10 on
                                                                                            20
                               Channel 10 off


              0   10    20      30    40       50    60     70    80     90   100           0
                                            Time (ns)                                        0   5   10    15    20      25     30      35      40        45
                                                                                                          SwitchingTime (ns)
                  Current source rise time can be designed for application.
                  Inherent laser limit is in ~ 2-10 ns range.
                  Thermal transients can complicate rapid switching.
                                                                                                 38




SG-DBR Laser with Integrated SOA
 High Power Widely Tunable Laser:
                                           20
                                                                    40 dB
                                                                                        13dBm


               Fiber Coupled Power (dBm)
                                           10

                                            0

                                           -10

                                           -20

                                           -30

                                           -40                                          L-band
                                                 1570   1580    1590      1600   1610
                                                          Wavelength (nm)
>100 50 GHz ITU Channels
Fiber coupled power = 13dBm = 20mW
SMSR > 40 dB
SOA: Power leveling, blanking, and VOA w/o degradation of SMSR
Channel switching time (software command verified channel) < 10 ms
                                                                                                                                                                39




      RIN & Linewidth Dependence on Power
                              RIN vs. SOA Current                                              White FM Noise Density vs. λ
              -130                                                                         5                                          13

                                                   SOA Current
                                                                                                                                      12
                                                   40 mA, 5.7 dBm                          4
              -135                                 60 mA, 7.5 dBm
                                                   80 mA, 8.6 dBm




                                                                                                                                           Output Power (dBm)
                                                   100 mA, 9.2 dBm                                                                    11




                                                                         Linewidth (MHz)
                                                   120 mA, 9.8 dBm                         3
RIN (dB/Hz)




                                                   150 mA, 10.5 dBm
              -140                                                                                                                    10

                                                                                           2
                                                                                                                                      9
              -145
                                                                                           1
                                                                                                                                      8


              -150                                                                         0                                          7
                     0        2000    4000      6000    8000     10000                     1525 1530 1535 1540 1545 1550 1555 1560 1565
                                     Frequency (MHz)                                                       Wavelength (nm)

               ■         RIN is only weakly dependent on output power (SOA current).
               ■         Linewidth is less than 2.5 MHz across all wavelengths
                          −   Scales with Laser Power as expected.
                                                                                                         40




                          SGDBR-SOA-EAM
                          Transmission Characteristics
                          5                                             1528 nm   1560 nm
error rate, dB




                                  OC-48
                                  Std. SMF
                          4
                                                        350 km
                          3
-10




                                                                        1540 nm
Dispersion Penalty @ 10




                                                                                  PRBS 231-1 at 2.5 Gb/s
                          2                             275 km
                                                                                  4th order Bessel-
                                                                                  Thomson filter
                                                                                  SONET mask with 25%
                          1                                                       margin
                                                        200 km                     Unfiltered 2.7 Gb/s
                                                                        1550 nm
                          0
                           1.53       1.54   1.55   1.56         1.57
                                       Wavelength, µm
Dispersion penalty at 10-10 errors/s error rate
for 200, 275, and 350 km of standard SMF for
38 ITU channels sampled across C-band.
                                                                                                                              41



SGDBR-SOA-EAM
RF-ER, Pave, & VOA Operation
                                6                                          15                              3 dBm   ~1550 nm
   Time Averaged Power (dBm)




                                                                           14




                                                                                RF Extinction Ratio (dB)
                               5.5

                                                                           13
                                                                                                           2 dB
                                5
                                                                           12

                               4.5
                                                                           11
                                                                                                           0 dB

                                4                                          10
                                     192   193     194     195       196
                                           Optical Frequency (THz)
                                                                                                           -3 dB
Ave. power >5 dBm and RF ER > 10 dB across C-band
Output power dynamic range of ~10 dB w/ small change
in SMSR and Wavelength (open loop operation)
                                                               42




OC-192 Operation of EAM
                         PRBS 231-1, Vp-p = 3V




     Integration technology compatible with higher bit rates
     > 10 dB RF ER across C-band
     Not optimized, improvements to come
                                                 43




MZ-SGDBR (UCSB)
                             Curved waveguides
                                  200µm




                   MMI Length:96µm

       Light Out

                                   Width: 9µm
                      Taper:20µm
                                                                                                                                             44




Extinction & Chirp: MZ-SGDBR (UCSB)

 • > 20 dB extinction                                                Chirp parameter as function of
 with 2V drive                                                       DC extinction curve for 550µm
                                                           2                                                             -5


 • Negative chirp when                                                                                                   -10
                                                           0
 increasing reverse bias                                                 Alpha 1525nm




                                                                                                                               Insertion Loss (dB)
                                                                         Alpha 1545nm
 ‘turns on’ modulator                    Chirp Parameter
                                                                                                                         -15
                                                           -2

                                                                                                                         -20

             ∆neff ( real )       2 ∆φ                     -4
 α chirp =                    =                                                                                          -25
             ∆neff (imag )        ∆αL
                                                           -6
                                                                                                                         -30
   Measured by the                                                                             (Power (dBm) 1525nm
                                                                                               Power (dBm) 1545nm
   Devaux method                                           -8                                                            -35
                                                                -4    -3.5   -3    -2.5   -2   -1.5   -1    -0.5     0

                                                                                  Arm #1 DC Bias (V)
                                                                                             45




MZ-SGDBR RF Performance: Lumped (UCSB)

 • BCB for low capacitance         10Gbit/s
 • Lumped drive– can improve      Eye 1015-1
 with traveling wave electrodes     PRBS



                                                          0




                                         Normalized S21
                                                          -2


                    BCB                                   -4


                                                          -6
                                                                   -4V Bias

                                                          -8
                                                               0     5      10     15   20
                                                                      Frequency (GHz)
                                 46




Contents
■   Why Tunable Lasers?

■   Basic Tuning Mechanisms

■   Examples of Tunable Lasers

■   Control of the Wavelength

■   Reliability Issues
                                                                 47




    Control Issues
■   Finding the desired channel
    − Look-up tables vs. channel counting?
    − Is global wavelength monitor required?
    − Must look-up tables be updated over life?


■   Staying on the desired channel
    − Is locker required to meet spec?
    − Is single knob control from locker sufficient over life?
                                                                             48




Generic Tunable Single-Frequency Laser
                                                                   Tunable
                   Gain                    Mode-
                                                                    output
                                Cavity    selection
                                phase       filter
                                 (∆n)       (∆m)            (∆L)

Mirror-1                                              Mirror-2

                                                            mλ/2 = nL
                                 Gain

           Mode-selection   Lasing mode
            filter (∆m)
                                                      Possible modes(∆n, ∆L)

                                                       λ
                                                                49




    Control comparison across types
 Laser          λcoarse      λfine    Amplitude      VOA
DFB Array/SOA  Varray(j)      T         Igain(j)     ∆ISOA
DFBs/MEMs     VM1, VM2(j)     T         Igain(j)   VM1, VM2(j)

SGDBR/SOA       Im1, Im2      Iφ        ISOA         ∆ISOA


Ext. Cavity      VMθ        VML, Iφ     Igain       VMshutter


VCSEL/MEMs       VM1         V*M1       Igain        --------
                                             50




Iolon Control Scheme for Ext. Cavity Laser
                                                                               51




     Agility Control of SG-DBR Lasers

Control Circuitry                        Wavelength Locking
                                         Mirror Control
■   DWDM DBR                             Power Control
    −   Power Control
    −   Temperature Control(fixed)
    −   Wavelength Locking
    −   Mirror Control (Locking?)

                             Light Out                              SG-DBR

■   DWDM DFB comparison                  SOA Front Mirror Gain Phase Rear Mirror
    − Power Control
    − Temperature Control
    − Wavelength Locking
                                 52




Contents
■   Why Tunable Lasers?

■   Basic Tuning Mechanisms

■   Examples of Tunable Lasers

■   Control of the Wavelength

■   Reliability Issues
                                                                     53




Wavelength Reliability
■ It’s not enough to just put out the right power in a single
  mode for a long time (old criterion)
■ Prior to end-of-life of a multi-channel DWDM source,
  power & wavelength must be in spec.
■ Intimately linked to wavelength control (or lack of it)
    − Finding the desired channel
        − Look-up tables vs. channel counting?
        − Is global wavelength monitor required?
        − Must look-up tables be updated over life?
    − Staying on the desired channel
        − Is locker required to meet spec?
        − Is single knob control from locker sufficient over life?
■   If look-up tables must be updated, how can this be done
    reliably?
                                                                          54




What causes the wavelength to change

                                              Tuned by mode-selection filter
                   ∆λ = ∆n + ∆L - ∆m          (via index or grating angle)
                    λ    n    L m
Tuned by net cavity index change
                                   Tuned by physical length change



Physical Causes, assuming a fixed look-up table:
∆n – Changes in internal temperature, Tint, or carrier lifetime, τc
∆L – Physical movements—solder relaxation, MEMs charging
∆m – ∆n of DBR, ∆θ of ext. grating, or MEMs charging
                                                                         55




    Critical issues for wavelength stability

 Laser              Variables in Table             *Critical ∆λ issues
DFB Array/SOA            j, Ig(j), T, ISOA             ∆n(Tint)   ∆Ig
DFBs/MEMs            j, Ig(j), T, VM1(j), VM2(j)       ∆n(Tint)   ∆Ig

SGDBR/SOA               Im1, Im2, Iφ, ISOA            ∆nDBR(τc)   ∆m

Ext. Cavity        VMθ, VML, Iφ, Igain, VMshut       ∆L(VM), ∆m(VM),
                                                     ∆n(Tint) ∆Ig

VCSEL/MEMs                 VM1, Ig                       ∆L(VM)

              *Requiring table update or global channel locator
                                                                    56




  Estimated Open-loop Wavelength Shifts
 Laser         Critical ∆λ issues   ∆λ @ EOLgain (No table update)
DFB Array/SOA ∆n(Tint)     ∆Ig         40GHz (10GHz/SOA feedback)
DFBs/MEMs     ∆n(Tint)     ∆Ig         40GHz

SGDBR/SOA      ∆nDBR(τc)     ∆m        <10GHz

Ext. Cavity    ∆L(VM), ∆m(VM),         100GHz (MEMs charging)
               ∆n(Tint) ∆Ig

VCSEL/MEMs          ∆L(VM)             1000GHz

• Only SGDBR lands on correct mode near EOL Open-loop
• Others require global channel monitor or the like
                                                                                                                   57



               Effects of SGDBR Mirror Aging:
               Measurement
       7.5                                                         0.09


                                                                   0.08
       6.5

                                                                   0.07
       5.5
                                                                   0.06

       4.5                                                                                            240 Hours
                                                                   0.05
rtBM




                                                                                                      576 Hours
                                                                                                      1080 Hours
       3.5                                                         0.04                               1416 Hours
                                                                                                      2088 Hours
                                                                                                      2304 Hours
                                                                   0.03
       2.5                                                                                            2640 Hours


                                                                   0.02
       1.5
                                                                   0.01

       0.5
             0.5     1.5       2.5          3.5        4.5   5.5   0.00
                                                                          0   1   2   3   4   5   6
                                     rtFM

                           t=96 Hours   t=2304 Hours


              ■    Corresponds to > 100 yrs of operation
              ■    Aging gives fixed amount of root current increase to provide a shift
                   in the “mode map” to higher current .
                                                                                                                           58




Very High SGDBR Wavelength
Stability and Reliability
                                3        2        1        0       1          2       3

                         104                                                                  ~ 106 Device Hours
                               MTTF ~ 350 yrs
                               MTTF ~ 350yrs
                                                                                              measured.
                               σ ~ 0.56
                               σ ∼ 0.56
                               Ea=0.55 eV, n=2
Mirror Life Time (yrs)




                               λFITS @ 25 yrs < 1                                             Very low Bragg Wavelength
                         103
                                                                                              Aging Rates < 0.5 pm/ year at
                                                                                              worse case.

                         102                                                                  Gain and SOA sections have
                                                                                              similar MTTF and failure
                                                                                              distribution.
                                                                Mirrors - Experimental
                                                                Mirrors - Least Squares Fit
                         101
                               0.1   1       5        20   50     80     95       99 99.9
                                                                                              OK for open-loop operation
                                             Cumulative Failures, %
                                                                                              no mode hops or incorrect
                                                                                              channels
                                                                                                                            59




    SGDBR Laser/SOA FITs vs. Time
                                                     250

• Open-loop failure rate vs.
time                                                 200




                               Failure Rate (FITs)
                                                     150
• Gain section determines
EOL                                                  100



• Closed-loop mirror control                         50

has also been implemented to
monitor any drift                                     0
                                                           0            5             10         15          20           25
                                                                                   Operating Time (yrs)

                                                           2*Mirror Failure Rate                  Amplifier Only Failure Rate
                                                           Gain Only Failure Rate                 Total Failure Rate
                                                                                                                                                    60




    SGDBR vs DFB Chip Reliability
                                                 10000
                                                                                   Use Current Density
■   Historically, DBR Reliability




                                    Lasing Wavelength Shift (pm/Year)
                                                                                                 2
                                                                                       (~4kA/ cm )
    WAS Poor…                                                    1000


■   Defects in the grating area,                                        100
                                                                               DFB EOL
    found to be primary cause of                                                                     *Mawatari, 1999
    DBR failure.                                                          10

                                                                           1
■   Improvement to re-growth                                                                           Agility, 2002
    (InP/InP) and minimal grating
                                                                         0.1
    area of SG-DBR, allow
    equivalent or better
    performance vs. DFB’s.                                              0.01
                                                                               1                       10            2
                                                                                                                                                 100
                                                                                           DBR Current Density (kA/cm )
                                                                         *Mawatari et al, “Lasing Wavelength Changes Due to Degradation in Buried
                                                                         Heterostructure DBR Laser”, Journal of Lightwave Technology, v.17, no.5 1999
                                                                       61




     Summary
■   Tunable lasers can reduce operational costs
■   Narrowly tunable versions have some short term
    inventory/sparing cost advantages but newer full-band types
    offer many further opportunities
■   Several configurations have emerged for current applications
■   Monolithic integration offers significant potential for reducing
    size, weight, power, & cost
■   Wavelength control issues still exist for many configurations.
    Look-up table updating and/or global channel monitors are
    necessary in some cases.
■   Reliability has been proven for the SGDBR version without any
    updating of the look-up tables or need for channel searching

								
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