Semiconductor laser diode produces stabilized optical frequency combs for telecommunications, metrology, signal processing and spectroscopy. October 22, 2010Posted by POP Section Chair in : Photonics and Optics (POP), Technology Transfer, Business Development and Entrepreneurism (TBE) , trackback
by Dr. Peter J. Delfyett, President, National Society of Black Physicists and University Trustee Chair Professor of Optics, ECE & Physics
A novel approach to generating a stabilized, phase-locked and coherent set of periodic optical frequency combs enables researchers to develop these sources at a fraction of the cost of conventional solid state and fiber based sources and thus facilitates their use in a broad range of information, sensing and measurement applications.
The use of lasers in communications, signal processing, test and measurement systems and spectroscopy has enabled many key advances over the past decades owing to the wide range of useful characteristics that accompany coherent laser radiation. Further advances are envisioned by using multiple lasers, each with differing wavelengths that allow parallel optical channels to increase the measurement and processing capability. Additional functionality is enabled if each optical frequency channel is phase-locked to the other optical channels, i.e., if the relative phase relation between each wavelength channel is well established and fixed and not drifting over long times . To achieve a set of frequency combs with a fixed phase relation, one can use a single continuous wave (cw) laser and modulation techniques such as amplitude or phase modulation to create sidebands . Further channels can be added through nonlinear optical interactions such as four wave mixing. In this approach, the characteristics of the optical frequency comb, such as frequency stability and linewidth, is completely determined by the cw laser and the electronic signals applied to the modulators. Alternatively, one can use mode-locked lasers to generate a frequency comb with a phase coherent relationship between each wavelength component. The drawback with a conventional mode-locked laser is that the frequency comb can drift owing to both environmental effects and background quantum effects, such as fluctuations in the background spontaneous emission in the gain medium of the laser .
Recently, Hansche and co-workers developed a technique that produces a stabilized optical frequency comb based on a technique that requires a mode-locked laser to possess an octave of optical spectra . A salient feature of this approach is that the frequencies of optical comb can be generated on an absolute frequency grid, i.e., the optical frequencies are precisely known to exist at exact multiples of the pulse repetition rate of the laser. A drawback, however is that the laser must possess an octave of optical bandwidth, and generally, the pulse repetition frequency is sufficiently low that it is difficult to have access to the individual comb components, owing to their relative close spacing. This feature of the close optical frequency spacing then makes the optical source difficult to employ in commercial applications, such as wavelength division multiplexed optical communication networks.
Our work has focused on developing an optical source that produces a set of widely spaced optical frequencies compared to the octave spanning approach so it becomes suitable for a broad range of communication and signal processing applications without the need of an octave of bandwidth, and with higher spectral purity and stability of the cw modulated approach. The approach is also self-referencing in that the optical frequency comb is reference to a secondary optical standard, such as a high-Q etalon. To achieve the wide optical channel spacing and high spectral purity, we engineer the optical cavity so that the generated optical frequency comb has wide channel spacing and simultaneously a very narrow linewidth for each “tooth” of the frequency comb. These characteristics allow the source to possess very low noise, and possess excellent spectral purity for each comb component. In addition, our approach can be performed using any gain medium, e.g., semiconductor, so that the produced comb can be placed anywhere within the optical spectrum, e.g., uv, visible, near infrared, etc.
To achieve a stabilized comb of coherent, phase-locked optical frequencies, we engineer the optical cavity to possess wide mode spacing and narrow linewidth. This is achieved by employing a nested cavity configuration that combines two cavities, where the main cavity has a finesse of 100 and a free spectral range of ~ 5.6 MHz, and a secondary internal cavity with a finesse of 1000 and a free spectral range of 10.287 GHz. The combined cavities generate a frequency comb with a spacing determined by the internal cavity, and individual narrow comb linewidths defined by the main cavity. The free spectral range of the secondary cavity also defines the laser’s pulse repetition frequency. The laser system schematic is shown in Fig. 1. The system may be regarded in two parts; the actively mod-locked laser cavity, and the Pound-Drever-Hall optical stabilization loop. A dispersion compensating fiber section of 3.5 meters is included to reduce the cavity dispersion. Active mode-locking is achieved via loss modulation using a Mach-Zehnder style modulator at 10.287 GHz. The driving signal to the modulator is obtained from an ultra low noise oscillator.
The purpose of the internal cavity Fabry Perot etalon (FPE) is twofold. First, the inclusion of the etalon allows only a single phase locked mode group, or supermode, to lase. Without the inclusion of the etalon, ~1830 interleaved supermodes will compete, and the resulting random fluctuations in amplitude and phase will disturb the output pulse train. This noise manifests itself in the timing and amplitude noise spectra as a series of noise spurs, called supermode noise, at multiples of the cavity fundamental frequency (5.6 MHz in this case). Also, the simultaneous lasing of different optical supermodes precludes the use of a single phase locked frequency comb with multigigahertz spacing. In the frequency domain, the FPE may be considered as a periodic bandpass filter that selects a single optical supermode. Without stabilization of the laser cavity, however, environmental influences will cause the optical frequencies to drift relative to the transmission peaks of the FPE. These frequency fluctuations will destabilize the mode-locking. The modes of the laser cavity are therefore stabilized to the FPE with the Pound-Drever-Hall (PDH) laser frequency stabilization method. The PDH stabilization loop uses the FPE to detect small changes to the optical frequencies of the laser to create an error signal that, after conditioning by a proportional gain-integration-differentiation (PID) controller, is fed back into a piezoelectric actuator to compensate for the frequency change. Thus supermode suppression and optical frequency stabilization are achieved simultaneously with a single intracavity FPE. The resulting performance of this laser produces a spectrally flat stabilized optical frequency comb of ~ 200 comb components on a 10.24 GHz grid. The individual comb linewidth is <500Hz with a stability of ~150 kHz, and has >50dB contrast. The generated periodic pulse train has an overall timing jitter (1 Hz to 100 MHz) of ~ 3 femtoseconds, with an intensity noise of 0.023%. To our knowledge, this is the lowest jitter multi-gigahertz stabilized optical comb source. Owing to its well defined stable optical comb and low phase noise this laser has applications in photonic analog to digital conversion, coherent communication, arbitrary waveform generation and optical clock distribution.
To realize the potential processing speeds and accuracy that photonics promises, the use of stabilized phase coherent optical frequency combs is a step toward that vision. Our work shows that the generation of stabilized optical frequency combs can be obtained with excellent stability without the need of octave spanning spectra with the cost effectiveness, electrical efficiency, and compactness of semiconductor diode lasers.
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