Optical generation of rapidly tunable millimeter wave subcarrier

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					       Optical generation of rapidly tunable millimeter
                       wave subcarrier
                                      Yifei Li, Amarildo J. C. Vieira and Peter R. Herczfeld

  Abstract - Several application, like frequency chirped lidar-           the optical heterodyning of two ringoscillators [6]. However,
radar system, opticdwireless communications and biomedical                so far these transmitters employ thermal or PZT tuning,
imaging, require optical transmitters that can generate rapidly           which are essentially slow processes. Inspired by the success
tunable, low phase noise and low intensity noise millimeter wave          of the mode locked microchip laser with the LiNb03 host, we
sub-carriers. Using an electro-optic Nd:LiNbOJ microchip laser            explored the feasibility of rapid tuning of these devices.
in an optical heterodyning scheme, we designed a simple, robust,
monolithic and low-cost optical transmitter, that can generate
                                                                                        11. THETUNABLE LASER CONCEPT
tunable sub-carriers from DC to 5OGHz. Furthermore, we will
demonstrate that due to the unique approach, whereby the
tuning and lasing occur within the same crystal, the dynamics                The basic microchip configuration is depicted in Fig.]. The
of the tuning process is fast.                                            single Nd-doped LiNb03 slab contains two identical lasers,
                                                                          which are mounted on a single substrate. The optical cavity is
 Index Terms - Microchip lasers, lidar system, Nd:LiNbO&                  formed by dielectric mirrors, which are deposited directly on
millimeter-wavegeneration.                                                the host crystal. The identical devices are pumped by
                                                                          semiconductor laser arrays. The length of the optical cavities
                         I. INTRODUCTION                                  is set to OSmm, which ensures both single-mode operation of
                                                                          the lasers and high pump absorption efficiency. Gold
   There are numerous applications like frequency chirped                 electrodes are deposited on the top and the bottom of the
Lidar-Radar system [I], opticallwireless communications [2]               lasers in the transverse direction. Ideally, in the absence of
and biomedical imaging [3], requiring optical transmitters                applied field, the two lasers emit at identical wavelength
that can generate rapidly tunable millimeter wave sub-carriers            (small differences in the optical output frequencies can be
with low phase and intensity noise. Semiconductor laser                   compensated for by a small DC bias). However, by applying
diodes are often limited in their performance due to their                a field to one of the lasers, its index of refraction is
inherently high noise. Compact, efficient solid-state                     modulated. This produces a shift of the output wavelength
microchip laser with high spectral quality and low noise [4]              and sets the stage for heterodyning.
shows great potential as an ideal optical transmitter for these
  A mode locked Nd:LiNb03 microchip laser has been
successfully demonstrated at Drexel University [5]. This laser
provided 20 and 40GHz modulated optical carrier, with
                                                                                                                   I     Electrode   I
nearly 100% modulation index, a measured phase noise was
below -1 10dBc/Hz at lkHz offset, intensity noise was below
-150 dB/Hz, and a maximum optical output over several tens
mw was recorded. The unique feature of this mode locked
microchip laser is host crystal, LiNb03, which has a large
electrooptic effect that allows for an efficient interaction
between the optical and millimeter wave signals within the
same crystal.
  While the mode locked Nd:LiNb03 microchip laser offers
very high quality millimeter wave modulated optical carrier,
its tunability is limited due to the mode-locking process,                              Fig 1. The duql microchip laser stmcture
which is inherently narrow band. Tunable optical transmitters
usually employ optical heterodyning technique. For example,
millimeter wave modulated high quality optical carrier with                 The gain bandwidth of Nd-doped LiNb03 was determined
tuning range from dc to over lOOGHz has been reported by                  to be 120GHz [7], which represents the upper bound of the
                                                                          tuning range. The monolithic configuration gives the system
  Manuscript received on April 18, 1999 The authors are with Center for   simplicity, compactness, stability, and insensivity to external
Microwavehghtwave Engeenenng, Drexel University, 32d & Chestnut           temperature fluctuations. It should be "pointed out that the
Streets, Philadelphia, PA 19104, Phone (215) 895-2914, Fax (215) 895-     only electrical inputs are for tuning.
4968, E-mail Yifeili@io ece drexel edu

      0-7803-5807-4/99/$10.00 0 1999 IEEE                             645            SBMO/IEEE MTT-S IMOC'99 Proceedings
                       111. TUNABLE   DESIGN
                                                                                                                  Laser Ihreshold & slopc efficient                    0.18
      During the design of the proposed tunable millimeter wave                               0:04,
    transmitter many issues were addressed. The laser design, the                                                                                                       .6
    laser tuning, tuning speedtL4,and noise reduction were
    investigated and are described below.
                                                    I    .                                                                                                            0.12
    A. Microchip Laser Design

      The laser output, P,,, of the 4-level system can be written
                                                                                                                                                                      0.06       !
    where the slope efficiency, O,,is defined as                                                                                                                      0.04

                        T                                                                                                                                             0.02
              0 s   =(-)vTvA~~Q~~~vOP,~
                      T+L                                                                       0.01      0.015      0.02
                                                                                                                               mirror tr.nsmission elficiency
                                                                                                                      the ou~put
                                                                                                                                 0.025         0.03         0.035   0.04

      The threshold pump power and the saturation intensity are                          Fig. 2. Threshold pump power and slope efficiency vs. laser output mirror
    given by                                                                                     transmission efficiency.

                                                                                         B. Tuning
                                                                                           The wavelength tuning of the laser is described by the
              I       hVL
                    --,                                                                  following relation:
       Table I lists the definition of the symbols used in above
     equations and the range of values used in the design.                               where ne is the refractive index of extraordinary wave, r33 is
                                                                                         electro-optic coefficient for the tuning configuration in which
                                                                                         both the applied field and the light polarization is along the
                                                                                         optical axis of the LiNb03 crystal. E is the applied electric
                                                                                         field, and foptical the photon frequency.
      Symbols                Definition                       Rangdvalues
                                                                                           A device thickness of d=OSmm yields a tuning sensitivity
                                                                                         of 40MHz per volt, which results in a practical tuning range
          T           Output coupling (T=I-R2)               From 0.01 to 0.04           of up to 5OGHz. We estimate that the thickness of the slab
          L            Round trip cavity loss
                                                         2   - exp(-2al)    - R,         can be halved, which of course would double the tuning
                                                                                         range. It should be noted that the short cavity length prevents
                      Absorption Coefficlent @                O01-005/cm
                              1 084 urn                                                  mode hoping.
                           Cavity Length                           0.5 mm
                       Effective Cross-section
                                                                                         C. Noise Reduction
                                                             1 - exp(-2apl)            To minimize the noise of the heterodyned beat signal an
                    Absorption coefficient (pump)              I .75-1 8 /cm         optical phase locked loop and optical injection locking
                                                                                     scheme has been explored. The proposed optical transmitter
                                                                                     configuration, shown in Fig. 3, employs an optical phase lock
                                                                                     loop to lock the relative phases of the two microchip lasers.
                                                                                     Since the output of the microchip laser has an extremely
                                                                                     narrow line-width (- several kilohertz), the phase lock loop
                                                                                     alone would ensure sufficiently low noise performance for
                                                                                     most application [7]. For applications that require extremely
                                                                                     low noise characteristics and large tunability, such as a
                                                                                     frequency hopped secure communication system, optical
                                                                                     injection locking need to be added.
                                                                                       The optical phase locked loop can be realized with MMIC
      Fig. 2 shows the computer simulation of the threshold
                                                                                     components. Specifically, the loop filter, mixer, preamplifier,
    pump power and slope efficiency as a function of the output
                                                                                     and high-speed photodetector can be integrated on a single,
    coupling efficiency. To ensure .a reasonable threshold the                       low cost MMIC chip. The power *amplifier,which is used to
    output mirror reflectivity is chosen to be 99%. The
                                                                                     drive the tunable microchip laser, is isolated to achieve the
    reflectivity of the pump side mirror is designed to be 99.9%.                    required high drive voltage.

                                                    .'         .                   646
                                                                                velocity (or index of refraction) into the mathematical
                                                                                formulation of the microchip laser resulting in a very
                                                                                complex nonlinear problem, well beyond the scope of this
                                                                                paper. A simplified approach is followed here. Let t,
                                                                                represent the round trip time in the laser, which is of the order
                                                                                of 6 psec in our case. The cold cavity decay time of the laser,
                                                                                7,a ratio of the roundtrip tide fo the cavity loss (-1%) is
                                                                                0.6nsec. If D x a n d the tuning range is limited, then the
                                                                                laser is expected to have enough time to readjust its lasing
                                                                                frequency and a nearly continuos frequency tuning is a
                                                                                possibility. Considering the linewidth of the laser to be of the
                                                                                order of a few kHz, we estimate a tuning rate of 10
                                                                                GHz/msec. This number is based on a cold cavity decay time,
                                                                                which represents a very conservative estimate. The actual
                                                                                dynamics of the laser comprises of a continuous sequence of
Fig. 3. Optical transmitter block diagram with an optical phase locked loop     small perturbations, which should result in a significantly
        & optical injection locking .                                           faster cavity relaxation time constant and tuning speed.

 The optical injection-locking circuit is composed of an
,optical isolator and a commercially available integrated                         A tunable high-speed optical transmitter has been designed
 optical phase modulator. The injection locking is achieved by                  an analyzed. The transmitter consists of two microchip lasers
 seeding the nth harmonic o f the modulating microwave                          co-located on the same Nd-doped L i m o 3 crystal. The
 reference signal.                                                              outputs of the two lasers are heterodyned to produce a
                                                                                tunable millimeter wave signal. The ultras-short cavity design
D.Tuning Speed                                                                  provides for single mode operation with superior optical
                                                                                spectrum quality, while maintaining good pump efficiency.
 The tuning speed of the optical transmitter, a concern in                      The monolithic configuration of realizing two lasers within
many applications, is determined by three factors:                              one microchip crystal makes the transmitter more tolerant to
                                                                                the environmental fluctuations. The several schemes of the
  the response speed of the tunable material to the tuning                      phase stabilization schemes have been investigated. The
  signal,                                                                       tuning sensitivity is 40MHz /volt and continues tuning range
  the time constant of the phased locking circuit, and                          is SOGHz. The tuning speed has been investigated. Thus we
  the laser dynamics                                                            can conclude the tunable optical transmitter provides a good
                                                                                solution     to     chirped   lidar-radar,    opticallwireless
  The electro-optic effect is extremely fast, of the order of                   communications and biomedical imaging.
several hundred GHz, and poses no speed limitations to the
device. To consider the other two factors we assume that a                                                   REFERENCES
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V=V,t.                                                                              Radar Tech to Aerial Lidar Systems for Enhancement of Shallow under
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