Semiconductor laser diode produces stabilized optical-frequency combs

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Semiconductor laser diode
produces stabilized
optical-frequency combs
Peter Delfyett, Ibrahim Ozdur, Mehmetcan Akbulut,
Nazanin Hoghooghi, Dimitrios Mandridis, Sarper Ozharar,
and Franklyn Quinlan

Phase-locked, coherent periodic optical channels provide a cost-
effective, competitive alternative to conventional solid-state and fiber-
based sources.

Over the past decades, the use of lasers in communications, sig-
nal processing, test and measurement systems, and spectroscopy
has enabled many key advances owing to the wide range of use-
ful characteristics inherent in coherent laser radiation. Further
advances are expected from simultaneous deployment of multi-
ple lasers spanning a range of wavelengths. This will allow gene-
ration of parallel optical channels, thus increasing the system’s
measurement and processing capability. Additional functionali-
ty is achieved if each optical-frequency channel is phase-locked           Figure 1. Optical-frequency-stabilized mode-locked laser. SOA: Semi-
to the other channels, i.e., if the relative phase relations among         conductor optical amplifier. DCF: Dispersion-compensating fiber.
all channels are well established, fixed, and not drifting over             I: Isolator. IML: Intensity modulator. FPE: Fabry-Perot etalon.
long times.1 To construct such a set of frequency combs, one               OPS: Optical-phase shifter. VOD: Variable optical delay. PBS: Po-
can use a single continuous-wave laser, combined with modu-                larization beam splitter. PC: Polarization controller. PM: Phase
lation techniques such as amplitude or phase modulation to cre-            modulator. Cir: Circulator. PS: Phase shifter. PD: Photodetec-
ate sidebands.2 Alternatively, mode-locked lasers can generate a           tor. PID: Proportional-integral-derivative controller. PDH: Pound-
frequency comb with phase-coherent relationships between all               Drever-Hall.
wavelength components. The main drawback of a conventional
mode-locked laser is that the frequency comb can drift because             comb ‘tooth.’ This approach leads to very low noise and excel-
of both environmental and background quantum effects.3                     lent spectral purity for each comb component.
   We have developed an optical source that is capable of pro-                We generate a stabilized comb of coherent, phase-locked op-
ducing a set of widely spaced optical frequencies and suitable for         tical frequencies by employing a nested-cavity configuration,
a broad range of communication and signal-processing applica-              where the main cavity has a free spectral range of ∼7MHz,
tions. Our approach is also self-referencing because the optical-          while a secondary internal cavity has a finesse of 1000 and a
frequency comb is referenced to a secondary optical standard,              free spectral range of 10.287GHz. This configuration generates
such as a high-quality etalon. To achieve wide optical-channel             a frequency comb with a spacing determined by the internal
spacing and high spectral purity, we engineer the optical cavity
so that the optical-frequency comb has simultaneous wide chan-
nel spacing and a very narrow linewidth for each frequency-                                                           Continued on next page
                                                                                                            10.1117/2.1200904.1588 Page 2/2

cavity, and individual narrow comb linewidths defined by the          Author Information
main cavity. The secondary cavity’s free spectral range also de-
fines the laser’s pulse-repetition frequency. A schematic of the      Peter Delfyett, Ibrahim Ozdur, Mehmetcan Akbulut,
laser system is shown in Figure 1. The setup consists of two         Nazanin Hoghooghi, and Dimitrios Mandridis
parts, i.e., the laser cavity and the Pound-Drever-Hall (PDH)        College of Optics and Photonics
optical-stabilization loop.                                          University of Central Florida
   The purpose of the internal-cavity Fabry-Perot etalon (FPE) is    Orlando, FL
twofold. First, inclusion of the etalon allows only a single-phase
locked-mode group, or ‘supermode,’ to lase. Without the etalon,      Sarper Ozharar
∼1500 interleaved supermodes would compete, and the result-          Northwestern University
ing random fluctuations in amplitude and phase would disturb          Chicago, IL
the output pulse train. This noise manifests itself in the timing
and amplitude noise spectra as a series of noise spurs, or ‘super-   Franklyn Quinlan
mode noise.’                                                         National Institute of Standards and Technology
   The simultaneous lasing of different optical supermodes also      Boulder, CO
precludes using a single phase-locked frequency comb with
multigigahertz spacing. Without stabilization of the laser cav-
                                                                     1. P. J. Delfyett, S. Gee, M. Choi, H. Izadpanah, W. Lee, S. Ozharar, F. Quinlan, and
ity, however, environmental effects will cause the optical fre-      T. Yilmaz, Optical frequency combs from semiconductor lasers and applications in ultra-
quencies to drift relative to the FPE’s transmission peaks. These    wideband signal processing and communications, IEEE J. Lightwave Technol. 24 (7),
                                                                     pp. 2701–2719, 2006.
frequency fluctuations will destabilize the mode locking. A so-       2. S. Ozharar, F. Quinlan, I. Ozdur, S. Gee, and P. J. Delfyett, Ultraflat optical comb
lution is provided by PDH laser-frequency stabilization, which       generation by phase-only modulation of continuous-wave light, IEEE Photon. Technol.
                                                                     Lett. 20 (1), pp. 36–38, 2008.
uses the FPE to detect small changes in the optical laser frequen-   3. S. Gee, S. Ozharar, F. Quinlan, J. J. Plant, P. W. Juodawlkis, and P. J. Delfyett, Self
cies to create an error signal that—after conditioning by a pro-     stabilization of an actively mode-locked semiconductor based fiber ring laser for ultra-low
                                                                     jitter, IEEE Photon. Technol. Lett. 19 (7), pp. 498–500, 2007.
portional gain-integration-differentiation controller—is fed back
into a piezoelectric optical-phase shifter for compensation. Thus,
supermode suppression and optical-frequency stabilization are
achieved simultaneously with a single intracavity FPE.
   The resulting performance produces a spectrally flat, sta-
bilized optical-frequency comb of ∼150 components on a
10.287GHz grid. The individual comb linewidth is <1kHz with a
stability of ∼150kHz, and has >45dB contrast. The resulting pe-
riodic pulse train has an overall timing jitter (1Hz to 100MHz) of
∼8.3fs, with an intensity noise of 0.038%. This makes this source
well suited for a broad range of applications in areas of opti-
cal communications, signal processing, metrology, and test and
   To realize the potential processing speeds and accuracy that
photonics promises, using stabilized phase-coherent optical-
frequency combs is a step toward that vision. Our work shows
that generation of stabilized optical-frequency combs can be ob-
tained with excellent stability and with the cost effectiveness,
electrical efficiency, and compactness of semiconductor diode
lasers. Future work will focus on addressing the stability limi-
tations imposed by the present cavity configuration.

                                                                                                           c 2009 SPIE

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